If you ever find yourself troubleshooting a VSTO addin that does not load then these steps will interest you.
- Do not skip the basics and check the registry at
HKCU\Software\Microsoft\Office\<Product>\AddIns\<Application>because if the LoadBehavior key is not set to 3 the office application will not even try to load it on startup;
- Enable error alerts popups by configuring an environment variable
- Enable logging errors to file by configuring an environment variable
- Pray for an error alert popup or for an error in the log file so that you can fix its cause.