Calling convention in making DLL

I'm trying making DLL which is compatible with Visual Studio 6.0 from Visual Studio 2010.
In case of Visual Studio 6.0, there was no calling convention in the properties but I was able to set calling convention to each functions respectively like following.

extern "C" __declspec (dllexport) int __stdcall MyFunc( int num );

So I could use __stdcall for the functions to be called from VB6 and __cdecl for internal used function.

However, there's calling convention option in the Project Properties > Configuration Properties > C/C++ > Advanced > Calling Convention.
If I choose one,for example, __stdcall, "unresolved exter variables"error occurs at internal function(__cdecl).

How to use Calling Convention in Visual Studio 2010?