Since the version 1.5 of GN4, add-ins are managed in two flavors:

•32-bit add-ins: to be used with 32-bit GN4 applications, on a 32-bit OS or on a 64-bit OS.

•64-bit add-ins: to be used with 64-bit GN4 applications on a 64-bit OS.

Therefore, before compiling add-ins or debugging in Visual Studio Express, make sure you've selected the appropriate platform in the Configuration Manager in Visual Studio Express.

See Select 32-bit or 64-bit platform.