Try bees' "royal food"...I don't know what is the word in Englisg...it's a natural product by bees...When I was a child i was too slim and for a period I've been taking this strange thing...It helps a lot and it's completely natural!
It's not a joke!

My best friend is naturally around 90lbs. and when she lost a lot of weight after getting her tonsils taken out, she drank a lot of milkshakes to gain it back.

O_o

Hmmm

tuna, salmon, flax, and walnuts. Good sources of protein include lean meats, fish, poultry, nuts, seeds and legumes. Healthy carbohydrate sources include fruits, vegetables and whole grains.

and water.

That is the route id go,good healthy omega fatty acids, as im assuming you don't want to eat unhealthy, just gain weight. some dietary supplements that weight trainers use wouldn't be all bad either.

Drink a lot of beer.

Well, you need to gain more calories than you burn , so eat
It's not easy, I have been in that situation after being sick.

Shakes are the right idea. You can drink calories a lot more quickly and easily than eating them.

Hopefully something as obvious as eating more carbs helps you out... but that's all i got. I have the opposite of your problem so I avoid most carbohydrates and they don't normally cause you to break out or anything! Also start watching either OZ or Weeds... I don't want to get off the couch for days after i start watching them... you're sure to put on a few pounds!

Yeah +1 on the beer. Ever since I quit that shit i've been losing belly fat. HERE TAKE SOME OF MINE.

What kind of smart ass reply were you expecting?

Get Married!! LOL j/k

That's when I gained weight though seriously! But truthfully my dad has cancer and he had dropped about 35-40 lbs and we had to put weiht back on him so we made him have a daily milkshake, and he had a lot of tortillas and other bread stuffs.