My advice to you is to ignore people that say you must eat more fat, burgers, fries, etc etc

Avoid junk food, you can increase weight with good clean food.

Just increase calories, eat more often, force your self to eat more.

Let's say you can only eat 50gr of rice, try to eat 100gr or 150gr.
if you eat 1 steak, start to eat 2 steaks.
Train hard, after workout eat some carbs so you recover quickly and protein for the muscle.

If you can't increase your apettite, eating yeast (i think this is how we spell), that will make your brain think you are still hungry and it will tell you, that you have to eat more.