To be honest, this may not be the best place to get info about how to gain weight- we're all trying to lose it! Well, most of us, anyway. Some of us have lost a little too much and could stand to gain a few pounds. Not me, though!
Probably the best thing for you to do is some protein supplements. Look for low carb, low sugar ones. They will give you some extra calories, plus the protein will help you build muscle. You can order samples of a bunch of different protein supplements at
Vitalady.com - Protein Supplements, Vitamins & Minerals, and Safe Snacks!. They are really geared toward weight loss surgery patients there, but I don't see why they would be appropriate for someone who wanted to use them to help gain a little weight, too.
Kelly