Drink beer you'll put on weight in no time. There is a reason it was called "liquid bread" in the early part of the twentieth century. Why you would want this, over, say, muscle mass I"m not sure.
I had the same problem as OP. I fixed it by eating most of a small can of peanut butter a day, and working out (lifting, not a lot of cardio) 4-5 times a week.