I bench with a belt. It doesn't make much difference really, I just find that it helps me keep a tight arch and not lift my ass off the bench. It's almost like a pressure gauge to make me aware of how tight my back and abs are. If it reduces my chances of failing a lift by 5% I will take it. That said, it does NOT help me lift more weight and I still do all non-comp. bench variations (such as Spoto press and 3 second pause bench) without a belt.
As far as positioning, that's up to you. I put it right over my navel and tilted slightly up at the back, whereas for squat and deadlift I would have it higher in the front. Just experiment and see what works for you. If you can't arch properly with the belt then either it's too tight, in the wrong place, or it just doesn't work for you.