Mono in VS2003 is a piece of cake, because there is an installer, and a community of users who have shaved all the problems away.

Sadly Mono in VS2005  is not that simple, and has given me considerable aggrevation, but I've now finally got a running add-in in VS2005. Following is an bullet point list of what I did to achieve this milestone:

1. Have mono installed, thanks P@ 😉
2. Download vsAddin2005 from http://anonsvn.mono-project.com/viewcvs/trunk/wintools/vsAddIn2005/ (Rob found this library)
3. Open the solution in VS2005
4. Open monoaddin.Addin and change the path in <Assembly> to the path you are using
5. Build everything
6. Copy all files from BinAndLib to the folder where your assembly is. (The code checks if they arein that folder. This surely most be a bug, because it would be enough to have them in the path.)
7. You need all files from the folders BinAndLib and reflib somewhere in your path, e.g. system32. (Could also add the folders to the path)
8. Start VS2005
9. Go to Tools->option, under environment->Add-in/Macros security add the folder which has monoaddin.Addin
10. Go to Tools->Addin manager…
11. tick all the options of the monoaddin
12. Open a solution and you can celebrate :o)