Women do not really need any machines to get abs.
Sit-ups are ineffective and can hurt your back.
Crunches are good...regular, reverse, obliques, bicycle...
Make sure you pull your stomach in when you exercise (and all day long, basically) so you get a flat stomach and not a protruding one.
A flat stomach also means the least fat reserves on top of it so it's not just about getting strong abs.