Poor Calorie Absorption and Exercise

Stating the obvious here but you have to consume more food than you need to in order to gain weight.  If you're maintaining your current weight with, for example, 1500 cal/day, then you'll have to bump that up to, say 1750 cal/day.  You'll eventually gain a little weight, then plateau.  At that point bump your intake up again to 2000 cal/day.  Repeat the process until you hit your target weight, and you'll have to continue at this level of caloric intake to maintain that weight.  Protein and fats will be your greatest ally, but don't avoid carbs.  I try to stay around a 60/30/10 % ratio with protein, fat and carbs.  Resistance training is a fine thing to be doing but it isn't necessarily going to make you heavier.  You should actually focus on lower body exercises.....your quads are the largest muscles in your body and consistently engaging them in resistance training will cause a more profound response from your central nervous system to focus repair/regrowth on the entire musculoskeletal system.  Squats, deadlifts and chest presses are the vital core group of movements you should be doing.  I generally have a day for horizontal push-pull movements, a day for vertical push-pull movements, and a leg day, spin bike on the alternating days.  I'd definitely encourage everyone to work themselves into a training and diet program and stick with it, it has been a profound positive aspect of my daily life for close to 20 years.