In my opinion, you need to fix a few things before people will start signing up. The top of your site's kind of a mess man. You have a monster image taking up the top 1/4 of the screen, and your google ad blocks are side by side which is doing all sorts of bad things to your actual forum tables. Check the attachment to see what I mean. The first thing you need to do is get it looking uniform on smaller, non-widescreen resolutions. 800x600 is kind of out the window nowadays, but even at 1280x1024 your site doesn't scale. (see attachment)
Another thing you might want to consider is trimming the number of subforums down a bit until you get more traffic and more posts.
The last thing - just be patient. Get your site looking good and visitors will come. My current site (after running two other sites to the 2M+ post mark) was very slow for quite some time, well over a year. Now I average 250+ online and ~50k posts a month. Stick with it, get the bugs worked out and it'll happen for ya.