Managed C++ Dll in C# code - How to unit test

We have managed c++ dll code used in C# code. Now while unit testing if i call C++ dll methods the return value is not as expected.Its because may be the dll is not loaded or not copied in output directory from where nunit is running. I added the reference to it in unit test project. How to load the c++ dll and dependent dll's of it while running nunit tests ?

Thank you for any help!