I got the mod installed, going single player first. multiplayer is greyed out though if i wanted to try it.

or is it enabled and i just dont know how to install it? I want to do an all nighter with some diablo boys soon!

anyone know how to enable it?

Belzebub is single-player only.
Multiplayer is being continued under alternate mod : Tchernobog (which is not yet implemented under one-click mods, hence one has to download & install it still manually)

So when you see Multiplayer grayed out - it is working as designed.