Just force the food in. I went from 165lbs to 225 in roughly a year and a half. Eating is practically a full time job for me. 7 meals a day (roughly every 2-3 hours) I eat until my stomach hurts. I am vegan though. I eat a tone of tofu and seitan for protein and also some potatoes. Also eat a lot of raw veggies such as bell peppers, tomatoes, and celery. A banana a day, a huge bowl of cereal every morning. It's not hard to put on some muscle, you just have to give it the fuel. Good luck! It took me a couple years to figure out just how much I had to eat. I also realized I couldn't possibly eat enough to do marathon running and body building. one or the other.
Evader is right. You just have to eat. This is a dream for Endomorphs because we have a huge appetite by default. By all means stuff your face, lift heavy, and get tons of rest. I also recommend L-Glutamine.