OK, next thing then: check that in that path, there's a file called "PInvokeDotNetAddIn.AddIn", and also "PInvokeDotNetAddIn2005.dll". The DLL should be version 188.8.131.52.
Also, what edition of VS are you running? Do you have the .NET 3.0 or 3.5 add-ons installed?
Developer, Red Gate Software Ltd