Eat lots and lots of high calorie food such as nuts, cheese, legumes, milk, meat. If for some reason you have trouble eating try a meal replacement drink or milk shake. I drink about about 2 a day and it's really helped to add some pounds. Also weight trainig might be something to think about to help you put on muscle mass. If you choose to do this I recomend finding a trainer rather then doing it on your own.
You could try milkshakes adding ensure in addition to your normal diet. You could also try to eat more carbs, pizza, pasta, etc
I hope you find an answer