6 pack comes from nutrition...if you're already lean but lack the muscle than use weighted ab movements or exercises that you can't do more than 12 reps with...i would suggest hanging leg raises, dragon flags, overhead side bends and the like...regular floor crunches are pretty much useless so you want to do exercsies that tax the stabilisers at the same time...all that being said if your doing enough Deads, Squats etc you should only need nutrition modifications anyway