If you are eating and eating and still not gaining weight, then you aren't eating enough. There is no secret here, you simply have to put more calories in than you burn. I'll bet you have no idea how many calories you eat in a day or have any idea how many calories you burn in a day. Until you figure those two things out, you're not going to achieve what you want to achieve. As I said, read the nutrition part of the site. It will make you determine those calculations and tell you what you need to eat - how many calories of each (carbs, protein, fats) to add mass based on how many calories you are burning. Until you do that and know for certain you are in fact eating more than your body is using - you're doomed to failure.

You can eat like a horse and still not be eating enough - at least that's what it may feel like. You need to get scientific with it and monitor what you're putting in your mouth.

Like I said, there is no secret here, it's simple math - calories in vs calories out - that's it.