It seems there's no way to actually put a forum, or users for that matter, into moderation. The only permission even mentioning moderation is "Follow Forum Moderation Rules", found under usergroup permissions. Permisions for moderation are missing from the forum node, specific user, etc.
I'm not sure what was intended, but I'm guessing it would be similar to VB with
Can Post Without Thread Moderation
Can Post without Post Moderation
or, a general "Require Moderation" switch.