Nice forum, I didn't realize ning forums could look assessable, most I've seen are dark themed and messy. I recommend advertising this on your blog because as your blog grows more and more will want to join. Also, I'd recommend removing the 12 year old part of the subtitle of forum because, 1st you're reminding wordpress and Ning that you're not at least 13 yrs old and wordpress might not "care" but Ning might.
Forums take a while to build, I joined a new electro music forum when is just started and I was contacted through last.fm to consider joining. Maybe you should go on some laptop social networking site you're apart of and ask people if they'd consider joining.
You're going to need to create a forum rules thread thats stickied and maybe some "introduce yourself" type thread.