If you can't see your abs, it's because
1) they are covered by bodyfat
or
2) the abs aren't sufficiently developed
or both.
To test #2, tense your abs a little. Can you feel the distinct bumps and ridges of your abs - like 6 little cobblestones? If you answered "no" to #2, you need to further develop your abs.
Some people will dispute this, especially men who tend to already have pretty strong and developed abs from other physical activites. Women on the other hand often have weak and undeveloped abs. And those will never look like a six-pack, no matter how low the bf.
My midsection used to be concave - no abs to speak of even though my bodyfat was really low. Doing weighted resistance exercises helped bring them out. Once they came out enough, I cut back on the weighted resistance, because the abs are just like any other muscle and will grow if you keep upping the resistance. Once your abs are sufficently developed, then it's all a matter of bodyfat and diet.
Since your overall bodyfat is low and you just harbor it in your abs, you can try topical Yohimbine HCI. I've used Yohimburn with great results on my butt (my stubborn fat storage area). There are lots of posts about Yohimbine/Yohimburn in the archives.