The ArcObjects SDK for the Microsoft .NET Framework installer for ArcGIS 10.0 does not recognize the Express Edition of Visual Studio 2010, so it does not install the templates and snippets.
Add-In development would be a pain without the templates. But I imagine if you manually added the proper references, classes, configs and settings one could get Express 2010 to work. In fact, I was planning to work on that, but haven't found the time...