Every ab exercises takes take of the whole ab region, but you can target the "lower abs". I do leg raises for this. To get an 8 pack you need a LOW body fat percent. I do 45 mins of direct ab workout every other day, I jog whenever my legs are not sore and I eat as healthy as I can. I have been doing this for a year and 3 months and the lowest section of my abs is really starting to get definition. Don't give up and good luck.
To work out your lower abs, you can do the incline reverse sit-ups. These will help tighten the muscles to form the packs that you want around your stomach area. You can get the instructions on how to do them on http://www.nowloss.com/best-lower-abs-exercise.htm.