| sit-ups are ok, but crunches are better.
however, to answer your question: a lot of people HAVE a six-pack already, but until you lower your bodyfat % they simple won't show through. Men typically have to hit single digit bodyfat percentages before their ab muscles show.
Also, some people just don't have genetically large ab muscles, so you may never have a visble six-pack. Some also only have 4-packs, or 8-packs.
Typically the diet has to be pretty strict and training for a cutting phase to make these muscles show. | 
