In my experience it's not difficult getting management to understand what refactoring is. The difficult part is justifying investment in refactoring vs. investment in new feature X that could make the company's future. If you are proposing refactoring as a project, this is what you're up against.
To sell refactoring as a project, you need to speak business's language, and that's a language of metrics and dollars. Show them how expensive it is to add features to your application. Explain concretely, if you can, features that have hitherto been impossible or infeasible to attempt because of the current state of the code. If your project will result in performance gains or reliability, try to put a number on that gain and shoot for it. Give them concrete goal that you're shooting for that they can support. And, most importantly, do this hard business analysis yourself! I suspect many engineers often dream of stopping the company for 3 months so a bunch of refactoring can get done. Suppose you did that. It'd feel great for the first month, but it's not going to feel great if the company tanks after that because the customers' needs are not being met. Do the refactoring that needs to be done, but keep the project lean and mean.
Trust can be very important when selling a refactoring project. If you have a history of delivering successful projects, and have demonstrated to the business that you know what you're doing, you're much more likely to have management on your side when you say refactoring is necessary. Of course, if you were able to deliver successful projects with this app, you wouldn't be so desparate to refactor it, right? There's your dilemna. :-)
If you can't sell it like this, try a different approach: work refactoring into existing projects. A week or two of refactoring, maybe even a month on longer projects, can be presented as part of the cost of doing that project if the refactoring is actually related to that project. If you go this route, be honest with management if they want to know how you're going to implement feature X, but don't call out the refactoring as a separate line item if you can avoid it – make it a cost of doing feature X.
Finally, be assertive, and don't give up if management doesn't get it right away!
Hope this helps!