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

•Release: to be used when compiling the final version.

•Debug: to be used with debugging.

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

Moreover, before starting edit-and-continue debugging, make sure you recompiled as the debug version.

See Select Debug or Release solution configuration and platform.