Yes, that can be fixed. If you don't have the manual, you can go to the proform website (do a search) and actually download the manual.
Before tightening it though, you might want to make sure the underside of the belt (between the belt and the deck) is lubricated with a good quality wax to lower any friction, which might be aggrevating the situation. Just tightening it will cause more wear on the belt and shorten the life.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix gym/fitness equipment.