From time to time I run into this problem with Visual studio 2008 (and 2005). An add-in is installed (3rd party or custom). The path to these add-ins is in the visual studio settings. When I start visual studio and load a project the add-ins are not installed and do not show-up in the list provided by the add-in manager.
I noticed this most recently with Reshaper not loading. What I want to know is what steps can we perform to diagnose why our add-in is not loading. FWIW, we tried using devenv /log and there was nothing in the log that indicated a failure and nothing in the log indicated that it even saw our add-ins.
Example: I installed dotTrace and resharper. DotTrace loads, resharper doesn't. I then installed testdriven.net and it also does not load. I uninstalled resharper, still no dice, but dotTrace consistently loads. Next step is obviously to uninstall dotTrace, but I'd prefer to have a clue about what's happening to short circuit the add-in process.