Does using a rowing machine help flatten your stomach?

Any cardio will help. Rowing machines are great because they hit your upper body, lower body, back and core. Treadmills just target lower body... The machines and situps will absolutely help tone and develop your ab muscles, but will not really "flatten" - if by flatten, you mean trim excess fat. Only cardio will significantly impact that. Do the rowing machine and stair climber religiously and you'll be good to go.