My son, who is 6'4" and weighs 214 lbs., was always tall and skinny. Ate anything and everything and never gained weight. Then he started lifting weights and put on about 50 pounds. He eats 7-8 times a day (LOTS of protein) and does heavy (300+) weight lifting six days a week. Of course, it's taken him a few years to get to this point; but he too, has that "hummingbird metabolism," so it can be done.
If you want to gain more weight then you should increase your daily calorie intake. Make sure that you burn fewer calories than the ones you gain by eating a high calorie diet. You should also keep track of your weight trends and get a fitness trainer.