If you have functions in a dll written in C++ that you want to access from C, you should declare these functions with C linkage instead of C++ linkage. Unless otherwise specified, the C++ compiler uses C++ type-safe naming (also known as name decoration) and C++ calling conventions, which can be difficult to call from C.
To specify C linkage, specify extern "C" for your function declarations. For example:
extern "C" __declspec( dllexport ) int MyFunc(long parm1);
More help can be found here.