Club Kryptonite is definitely a happenin' place. I wasn't as big a fan as I thought I would be, but it was cool. It was really packed the night we went, so I don't think I enjoyed it as much as I could have. Website: http://www.club-kryptonite.com/index2.html.
I would say if you do go, go a little bit earlier if you actually want to get in some dancing, because shortly after we got there, it started to get way too packed. Packed so full that you couldn't even move to dance. Also, invest in a Myrtle Beach VIP Card! It gets you into all the clubs for either free or discounted!