Look in the workout forum for a good mass builder.. and as redmeat said, concentrate on compound heavy movements, start your workouts with these.
Bench/Military/Deadlift/Squat
If you can't gain weight, you're not eating enough plain and simple. I don't care how high your metabolism is, unless you have a serious thyroid problem you WILL gain weight if you eat enough. Try to get 300 grams of protein per day, 300 grams of carbs, and maybe 150g of fats. Its a lot of food, split it into 6 or 7 meals per day. Eat what/when you have to, not when you want to. Sure, you might puke sometimes from all the excess food, it sucks, but it happens. Don't forget a PWO shake containing protein and dextrose in a 1:2 ratio... get yourself a good protein, I like Optimum Nutrition 100% whey myself.