Everyone has ab's it's just a matter of how much fat is covering them. It's going to depend on how badly you want them, but you don't need any machines to get them. As mentioned above it's diet, the rest is cardio.
To maintain where I'm at means a very strict diet, and 5 days a week of running five miles or more. I couple this with an aggressive weight routine to keep my stomach flat. I can get into some specifics but it was mentioned above to go to Menshealth, that's the place. They also have an excellent on-line trainer program that will give you the exercise routines and the meals.
http://www.mhpersonaltrainer.com/uof/mhpersonaltrainer/35605/