Biggie,
Plug your numbers into one of those formulas (they all come pretty close to teh same amount) and you'll find pretty close to how many calories you need to gain weight.
Just simply eating here and there isn't the same as having a certain number of calories to eat and meeting them. You really should only be shooting for an extra pound a week, two if you feel adventurous but then you'll be packing on extra fat rather than LBM.
I don't know what to tell you about the discouragement; if you haven't gained a pound since last week and you were ready to give up, that's up to you. But not gaining a pound in a week is hardly worth getting upset about. If you want to gain, you're going to HAVE TO eat 3500 EXTRA calories a week, that means at least 500 extra a day.
If you want to add calories-go buy some almonds, pumpkin seeds, granola, and sesame seeds in bulk.