Your VSTA Project template includes a reference to your proxy assembly. If the end user manually deletes the proxy assembly reference, they will not find the proxy assembly listed under the .NET tab.
To add your own assemblies to the .NET tab
Create a new registry key (for example, one called ProxyAssembly) beneath either of the following registry keys:
HKEY_LOCAL_MACHINE\Software\Microsoft\.NETFramework\AssemblyFolders
HKEY_CURRENT_USER\Software\Microsoft\.NETFramework\AssemblyFolders
Set the new key's default value to point to the folder that contains your assemblies.
For instance:
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\.NETFramework\AssemblyFolders\MyProxy]
@="C:\\Documents and Settings\\gdepue\\Desktop\\VSTA2.0Samples\\ShapeAppBasicMFC\\Proxy\\bin\\Debug"
If you have Visual Studio/VSTA IDE open, you must close it and then launch it again for the change to take effect.