Although you're asking for communications advice, what you're really talking about is a hidden agenda in your specific situation. Bosses with hidden agendas are a tricky and sometimes dangerous thing. Sometimes that agenda can be you, sometimes it can be something else. Your task is to get them to admit what the agenda is, while keeping their feathers unruffled.
You'll need to prepare, both physically (with supporting paperwork and - God Forbid - visual aides) and mentally. If you're lucky, just getting some mental preparation will suffice. If you're not, you may be faced with generating performance charts, etc. Still, failing to prepare is preparing to fail.
In your preparations, target specific things, and keep the list small and manageable. If you have more than 5 things, you're asking for trouble, the average person handles only 3-5 simultaneous events/concepts at a time, and you don't want to vex them with overload.
You need to set aside time. At least 15 minutes; if you get done sooner, it will be seen as a positive thing (we completed the meeting early and went on to other things, good job!). Warning Sign: you can't get more than 5 minutes, or worse, they think that the 5 minute manager means everything is answered in 5 minutes or less. This is a Bad Sign(tm), really bad, and you will not be able to break through your work issues because your boss has more or less erected a permanent barrier. I know this sucks, but if you can't get a time slot with them alone, take your lunch with them.
You need to get your boss to agree to stop moving around at this time. I mean, literally get him/her to sit still and listen, and not be distracted by telephones, other subordinates, paperwork, deadlines, etc. Close the office door and keep it shut. Distractions are the unintended ADHD syndrome of bosses everywhere, and even a missed cell phone call can break a train of thought, leading to "aw heck I don't care, just get back to work" as an excuse to get out of the meeting. The whole point of getting time with them is to get their attention, their focus, on what you are trying to convey.
Once you are ready to communicate effectively, keep one rule in mind: only speak when it's absolutely necessary. Unless it's absolutely needed to confer the information your boss needs, or if it's needed to keep the communication flowing, don't even bother openning your mouth. Samuel Clemens was remarkably spot-on in this estimation: "It is better to keep your mouth closed and let people think you are a fool than to open it and remove all doubt."
During this time, you need to (politely!) approach him/her and breach the topic. You will want to do so in a non-accusatory fashion, i.e. you want to do so without generating the feeling that anyone - you or your boss - is at fault for the situation. Rather, you'll be presenting the issue in a manner that encourages teamwork. You want this to be a positive impression and you will want to keep this upbeat. Even the slightest hint of negative can (and often will) turn against you. I cannot give this enough emphasis. Negative verbage, and even negative emotions, are weapons in the hands of a bad boss with a bad agenda that targets you. Do not give them this, it will devolve into an opportunity to use any of this against you. Yeah, you'll wanna puke with the constant "sunshine and pony" crap, but trust me on this, it's nigh-impossible to put the thumbscrews to you if everything they try to use against you is a positive. Hopefully that isn't the case and it's just a situation where there's a misunderstanding; and this meeting will set you both on the right track.
Avoid technical jargon, unless it's something they're going to hear over the long term. Think of this: remember when "cyberspace" was Hollywood's trendy buzzword? How would you like it if your boss latched on to this term and used it for the next 3 years? Wouldn't you cringe less and feel better when they speak, if they used the correct term(s)? This is the corollary to the "Speak when needed" maxim above - everything you say will be under a brilliant spotlight for a few moments, and verbal gaffs will stick around long after...
Assume that they don't know what it is you know. Assume that they also may harbor fear, resentment, indifference, or any one of thousands of other personal or political agendas. Also consider that they may be filtering what you say through those agendas.
Try and get them to clearly define what they consider to be progress. It might be something entirely different from what is called for in reality, and that will touch back on the personal/political agenda just mentioned. If their definition sounds even slightly vague, do not let it slide, politely ask them to clarify or provide additional details, what you want to accomplish is to get them to admit what their hidden agenda is in a polite manner that doesn't threaten them.
After it is sparkling crystal-clear what their agenda is, be sure to (politely) repeat it back to them, i.e. "...so just to make sure I have it down pat, you said you want such-and-such accomplished by means of xyz within a given timeframe that you specify, correct?" Repetition back to them will give them a sense that their communcation was effective and provide a positive feedback. Bosses like this because they feel their intentions are conveyed effectively, so this is a good thing(tm).
This should give you the critical insight you're after. You'll have a better understanding of their expectations - and like it or not, part of your job is to set expectations. If their expectations are unrealistic, or completely disconnected from the reality of the situation, it falls to you to provide realistic, clear, informative feedback that focuses on what they need to accomplish their tasks, not yours. If that feedback shows that their expectation is unrealistic, they'll quickly re-adjust it on their own. Sometimes, all that's needed is exactly that - a re-assessment on their part of what is really needed and what's really going on. The goal here is to start and establish an unbroken chain of communication between you and them, and that communication will only help everyone out over the long run.
If by this point you can't get a sense of what it is that they are after, then the situation is truly dire and you're not in a good working situation at all; either the boss is incompetent (either by lack of effective communication to their subordinates, or they are not even close to competent in the field they are working in), or they have a hidden agenda that is adverse to you. Either way you're on your way out in the long term. Thank them for their time, assure them that the meeting was well-received and informative, and drop the whole thing. In the meantime, stick it out and do the best you can, but start looking (discretely, on your personal time) for work elsewhere. I truly hope this isn't the case, but I have been in this situation at least once, and while uncomfortable, it's pretty clear you're better off just getting out while you can...
After this point, you're on your own. Good luck.
