I'm running into an issue with VB.NET 2010 and ArcGIS Add-Ins. After I create an add-in, compile, and start debugging the add-in will not install. I have to manually install the add-in using ArcMap or by double-clicking on the add-in with Windows Explorer.
Based on my experience, as long as ArcMap.exe is set as the external starting program in the project's debug settings page, when you build the solution, the add-in will be automatically put in the C:\Users\<user name>\Documents\ArcGIS\AddIns\Desktop10.0 folder. ArcMap can automatically discover add-ins from that folder and list them in the Add-In Manager, but you do still need to manually drag the add-in button/tool into the toolbar.
This is definitely strange behavior. I've tried the solution from the following thread here, and I also tried editing the registry as stated in the help to load all add-ins regardless of whether or not they have digital signatures. Neither have worked. I'm installing Service Pack 1 for Visual Studio 2010 to see if this will make a difference.
Sometimes I have problems concerning Visual Studio Debug mode. In those cases I delete all files in the debug folder of the visual studio project (in case my deployment method is debug in visual studion, for release of course delete all content of that folder). And I delete the AddInFiles under C:\Users\<user name>\Documents\ArcGIS\AddIns\Desktop10.0 folder (like Shuping posted).
Then I manually built and deploy the project. I will have a look if the files are deployed as expected (once in the debug folder and the Desktop 10 AddIn folder as well). How is it with your deployment. Are the files deployed correctly?
If this was successful I start the debug mode and in most cases this works. If not I try the same procedure several times, restart the programs and/ or the mashine. After some tries my problems are solved.
Regarding the registry settings. Try also to delete all registry settings of the advanced concept. Also take care of wow64node and x86 registry settings if you are working on a x64 system. The registry settings are not working in both although in both are registry entries for ArcDesktop10. But I do not know at the moment which one was the correct.