Do it a lot and with gusto would be my suggestion. I know some people who have DDR at home and do it pretty regularly -- not a lot every day, mind you -- and have put on a few pounds since getting the pad.
How's the diet (food consumed, not fad-type thing)? That could make a big difference, too.
Oh, and make sure you're getting enough water. If you want to break down bigger products (fats, protiens, carbs) the metabolic process involves hydrolysis, so you're going to prevent the breakdown of these molecules if you aren't properly hydrated.
P.S. -- Good luck, man! We gots your back!