Peanut Butter and whole milk.

Add 500 calories (clean; don't go around eating donuts all day. Not every calorie is the same) to your diet every day. If you're not seeing the kind of weight gain that you're looking for add another 250-500 calories.

Obviously you'll need to workout hard, otherwise you'll just get fat.

Now, onto the 6-pack. Leg raises, crunches and side bends (www.shelterpub.com/_fitness/_weight_training/dt-03.gif). But the only way your abs will show is if your body fat % is really low, so cut some weight after you're happy with your weight gain.