Ask Mr. Robot is showing an extra socket on [insert my favorite belt slot item here]! It does not have this socket in-game!
… but it COULD!
We show an extra prismatic socket on most belts for WotLK. You can buy a belt buckle from a blacksmith or the auction house to add this extra socket.
Sometimes you might not want the added expense on lower level gear. Set the Belt Buckle Min Item Level setting to the lowest item level item to which you want to add a belt buckle.