when you got to
and select a forum to edit, then, on the add/edit forum tab, you should see the fields »moderator« and »moderator group« (at least in Tiki 6.3 or 7.1 – what is the version you are using, btw?).
I guess what you're looking for is the field »moderator«, where you enter the user name of the user who is to moderate the respective forum.
If not a single person, a forum may be moderated by a group of people, say, the »Online Newsdesk Group« or something.