Ignore trying to gain weight, eat well, eat alot and work hard. It will come.
Good advice, the last thing you should focus on is the scale. Don't weigh yourself more than once a week, and always at the same time of the same day after similar eating habits. Just drinking a glass or two of water an hour before weighing yourself will move the scale so if you really want to be accurate the routine must be the same. Your weight isn't even a great indicator of the muscle that you're gaining because you will be losing fat as well. What you really want is a flexible tape measure so you can record all your measurements now and then again on a regular basis. You will find times where the scale will drop but your legs, arms, and chest will grow while your waist will shrink. Before and after pics of your definition will also help you track your progress. The scale is only one unit of measurement and it can lie to you.