How can I get a few inches of my waist?

I'm 22, 5'6 and 122lbs, I’m happy with everything on my body except my waist, it's around 30". I’ve been going to the gym 3-4 times a week, treadmill for 45 minutes with a 10% Incline and 3.5 - 4.0 miles an hour. I also do the abdominal machine, doing 50lb weights about 50 repetitions. What can I do to get the fat of my stomach and then build muscle and get toned?