I would say that if this forum script doesn't have code to add categories, it is most definately not at a usable state yet. Perhaps you should be looking into a finished forum script.
Unless it can do this. And you just can't figure out how. In which case maybe you should be looking into a more user friendly forum script. I'd suggest http://www.simplemachines.org/... I find it a good balance between light and featured... and it's pretty user friendly.