In terms of gaining mass it is actually fairly straight forward (in theory!):

If your calorie intake for the day is less than what your body uses in a day, you will lose weight.

If your calorie intake is more or less the same as what your body uses in a day, you will stay are more or less the same weight.

If your calorie intake per day is more than what your body uses in a day, you will gain weight.

Where a lot of people seem to come unstuck (usually while trying to lose weight but possibly while trying to gain weight too) is that they miscalculate the amount of calories they consume in a day. So something for your to consider is to keep a food diary for a while. See what you are eating, and record the number of calories for everything you have eaten that day. Add them up at the end of the day. If you can't find the calorie information on the food packaging, try looking it up on a food calorie website. These are usually best guesses, but they will give you a rough idea:

http://www.caloriesperhour.com/index_food.php

An average adult male uses about 2000 calories just moving about everyday. If he exercises, then of course he is going to use 2000 calories a day plus the calories he burns doing exercise.

What I would suggest is keeping a food diary. If you aren't gaining mass and are eating rougly the same amount of calories each day, then you may want to consider consuming more calories. I would encourage you to eat healthy, whole foods to add more calories. Don't try and bulk up with ice cream ha ha!

One final note on supplements and powders: So far as I am aware there is nothing you can get in any protein powder or meal replacement powder that you can't get in good wholesome food. If it were me, I would try to get my bodies nutrients from food rather than powders. I know this isn't always possible or practical, but I would make the effort to eat properly first and foremost.

As for building mass as muscle, well, there are no shortcuts for that I'm afraid. Good intensive strength and conditoning training will do that.

If you want ideas for weight/stength training to help you build up your body, check the strengh forum on here. It's pretty ace. If you need more help with weights/strength training, start a topic in there.

Good luck.
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"Let your food be your medicine, and your medicine be your food" Hippocrates.