Ab machines are completely worthless. They are one of the biggest lies in all of the fitness industry. You need NO equipment to get a solid ab workout.
No matter HOW much you exercise your abs, though, you're never going to get a 6 pack unless you lose fat (and probably a LOT of it), and you are never going to lose fat unless you burn more calories than you consume, and you're never going to burn enough calories doing ab workouts alone to make a significant dent.
Also, you cannot choose WHERE the fat comes from. Fat burnt doing ab work isn't necessarily going to burn belly fat. Your body will burn it wherever it sees fit to do so.
Do some crunches, planks, bicycles, scissor kicks etc. if you want ab strength.
And then run, or at the very least hop on a elliptical for 30 minutes or so. Cardio is where you're going to burn calories.
All that being said, consuming fewer calories is way easier to do than working them off. Going for a 3 mile run (for me at around 205 lbs) is going to burn maybe 500 calories. That's really not a whole lot considering how much work it is. Way easier not to eat those calories in the first place, though it doesn't do much to get you in shape.
All of these things (save for maybe the eliptical thing) are FREE. In fact, eating fewer calories can actually SAVE you money!!