First off, weight gainer is for people that are truly "hard gainers" and just cant put on weight if their life depended on it. So you need to take that into account when using it. Start off by making a shake that has only one scoop instead of the recommended 3. Use this once a day at night after dinner, also make sure that it is after you have worked out. Try doing cardio only about twice a week, and it shouldn't be too intense, something like basketball is ideal. As for your other meals, it is recommended by many to have at least 5-6 meals a day. Since this can be quite hard, try to get 4. Have a good breakfast in the morning consisting of cereals and eggs if possible, for lunch, tuna sandwiches are great, and make sure you eat chicken as often as possible. Dinner should be your biggest meal and should be consistent. I make sure I eat at least 1-2 more servings then it feels like I can take. You will get used to eating more eventually and will be able to build your appetite up. One common misconception is that you should eat anything and everything, this is often true for real "hard gainers" and I soon learned that I was not, and that I was in fact just gaining fat and my stomach was just getting bigger and not my muscles, so you need to stay away from anything that has a lot of unsaturated fats especially cheese, just stick to high protein foods.
Now, once you have done this for a good month and a half and are not seeing many gains or the gains have slowed down, it is then time to up the volume and start adding two scoops of weight gainer to your shakes. And trying to eat more food then you had before, adding another meal if possible.
Oh and make sure you are lifting heavy weights in order to shock your muscles. Some exercises are best left at the lower weights just because you don't want to sacrifice proper form for higher weights. However, whenever possible increase the weight from set to set this has proven to be effective for me.
Well, that should help you gain weight, if you have any more specific questions about the subject just keep posting here.