Failing to load a game DLL usually means it was blocked by something. Check your antivirus if it's interfering. Mainstream antivirus programs don't have this issue so I'd be a little suspect of yours.
It's also possible you failed to follow the installation instructions properly and there's no MBII.pk3 in the MBII folder, or the folder paths are entirely messed up, but you haven't provided any information regarding the install environment, so that would just be speculation.