In any case 2500 calories is probably too few for someone who is quite active - that's the approximate daily amount generally quoted for the average man - i.e. someone who drives to work, sits at a desk all day, maybe walks to the local shops or goes for a short jog every so often, and that's about it. If you exercise hard several times a week you probably want significantly more than that.
It's a fairly simple equation really. Broadly speaking, eat fewer calories than you burn - you lose weight. Eat more than you burn - you gain weight. Eat about the same amount as you burn - you stay about the same. If you want to gain weight you need to eat more!