I don't see how a machine can do it better than crunches/situps. If they're not working for you, try leg lifts. Lie on your back facing the ceiling, and then slowly bring your legs upward. Pause at the top, and slowly lower them down.
Stretching your hamstrings helps for this exercise. And, if you're still not feeling the burn, lift up your upper-body like you were doing a crunch half-way and hold that position. Do leg lifts.
And just a little piece of info. Everybody has a 6 or 8 pack (because of the way the abdominals are put together), but not everybody has visible ones because of the layer of fat in front. You could have the strongest abs in the world and not see a thing. Try to reduce body fat % by eating less fatty foods (potato chips, etc.) and drink water/milk/orange juice instead of carbonated drinks and coffee (and beer sucks long and hard for fitness). People say that eating smaller meals more often works, but I can't verify that...