As for the issue, the error message pretty much confirms what I suspected. You can only reference .NET assemblies and COM components in .NET projects, so your C++ DLL is almost certainly neither. As I said previously, you will have to use pinvoke to call the unmanaged functions exported by your library.

Where did you get this DLL? Is there any documentation with it that provides .NET examples? If not, you'll just have to research pinvoke to learn the principles involved, then apply those principles to your specific scenario. Basically, you declare a .NET method with the appropriate attributes that matches the signature of the unmanaged function you want to call. You then call your method and .NET will invoke the external function. You'll then need to deploy your unmanaged library with your .NET app. You might start here.