You probably shouldn't use it. Slendertone machines will never give you a flat stomach.
Do you have something like Lordosis? If your lumbar spine is excessively curved, then your organs will be pushed forwards, making you look pregnant. Slendertone will only tone underlying muscles, it won't give you a flat stomach.
If you can't do regular stomach crunches, you can do a crunch designed for people with back problems. You lie flat on your back with both hands under your lower back, palms facing the floor. Keep one leg straight and the other bent with your foot flat on the floor. Pull your ab muscles in tight, but keep breathing and then raise your shoulders off the floor. keep looking up, don't allow your chin to touch your chest, as this will cause strain in your neck.
Do 12-15 of these crunches, then straighten out the bent leg and bend the straight leg and repeat. Do the whole routine on both sides another one or two times.