One of the best exercises you can do for your abs is to exercise the rest of your body. Well defined abs are there but most often just burried under excess body fat. To reduce your body fat covering your abs work on developing lean muscle mass in other parts of your body. Lean muscle mass burns more calories at rest than does fat so tone up other areas including the large leg mucscles.
It also helps to watch the diet .... they don't call it a beer belly or a nugget gut for nothing
Hope this helps.