Okay, this happens to alot of people. First of all, you have to write a really long post or Quote someone for it to show. Little one-sentences don't deserve to have a huge sig under them. So yeah, you have to either make a lengthy post or just Quote someone in your post.
Also, what Forum Skin are you using? That's the way the site appears, and by default its Revolution. Its all black, isn't it? Yes, big signatures won't show all the way on Revolution skin. Go to Edit Options and scroll down to Change Skin. There are three options. I happen to use the Smash Blue skin, but that's just me.
Thirdly, if its too big, it won't show AND you can get infracted for it. I checked yours and your fine, though you need to change your Skin.
Hope this helped a little.