Man, I'm sick of people saying "diet is the only way to get abs"....
.... yes, it's important. Even essential. But it's not the only part.
Abdominals, like any other muscle group, require a combination of training and diet. I'm sick of people saying "Abs need diet and genetics. You don't need to train them."
One of my best friends back home, is the leanest person I've ever seen. His bodyfat is naturally probably under 5% and always has been. He has a super-charged metabolism. But, even with his extremely low bodyfat levels, he doesn't have a six-pack....
.... that's because the ab muscles aren't developed and therefore they don't stick out to the point of visibility.
1) Train the abs, make them bigger so they can be seen
2) Get rid of the fat obscuring them
I recommend hanging leg-raises, done like "half-up" reps.... in other words, the starting position is where your thighs are parallel to the ground, and you bring them up as high as you can, and then back down to parallel.... I believe this works the abs a lot better than leg raise movements where you go the whole way, as 60% of the ROM is mainly working illio-psoas and other hip flexor groups.