You don't know how lucky you are. You obviously have a great metabolism. (I WISH). I have been told by many people that body-build that the best way to increase body mass is to eat more protein (meat, cheese etc). I don't think fast food is the answer. You still need to have a balanced diet or you will put the weight on the wrong way.
Why don't you contact a GYM somewhere near you and speak to a dietician if you want to put weight on.
Ha........I think that is what I need