It seems that, currently, you need to select one of the "Forum" positions in order to get access to the conditions that allow you to set an ad for a specific forum - this is extremely bad design.
All of the forum-specific positions are, quite frankly, awful and I'd much rather place an ad in one of the Global positions. But if I place an ad in one of the global positions then I can't set the conditions to display the ad in a specific forum or forums. This needs to be changed.