The DLL you mention should be installed as part of the "TFS 201x Object Model" package. This package is usually installed as part of Visual Studio. I don't know why you're missing it, but you can install it separately. If you tell me which version of TFS you're using, I can find the package installer for you.
Most importantly, this is not a dependency that you need to lug around manually - it should come from GAC as part of the Object Model redistributable.