Richard has already described the first obvious error. The error messages with the inconsistent DLL bindings disappear as soon as the functions are declared with "dllexport".
After that there can still be problems with the path. While it is often sufficient in the development environment to specify a relative path, such as:
LibDll = LoadLibrary(TEXT("./../x64/Debug/MyDll.dll"));
it would be better to get a reliable path e.g. with GetModuleFileName().
There is still a small error at the top when loading. It should be of course:
suma = (op)GetProcAddress(LibDll, "suma");
if (suma == NULL) { }