It is probably a native binary and not a .NET assembly or COM object. You can only add references to .NET assemblies or COM objects. If it is a COM object then it should be registered on your machine. In this case go to the Add References\COM tab and find the COM object within there. The reference will be added automatically. For .NET assemblies use the Add References\Browse or Add References\.NET tabs to find the assembly.
For native binaries (i.e. C++ or C DLLs) you use DllImport attributeto reference the binary. It must be available in the application directory at runtime and no compile-time type checking is done.