The thing that motivates people into registering at your forum is the content - and as Slavik as already mentioned, you don't really have any.
You need to make at least 1 post in each forum so there are no empty ones, and then instead of spending money on ads, sit down and create some threads and start "talking" on your forums.
If you don't want it to appear like you're talking to yourself, create a few "extra" user accounts and use those in rotation so you're mixing-it-up a bit and creating converations.
Also, ask some questions - people love to share their knowledge and expertise, so pose a few retro questions and see if that helps to draw people in.
Bascially, put in some hard work.
You could also try registering at other forums in your niche (providing they allow links in signatures). Don't spam or troll, just get involved in a genuine way and share your passion, and let people explore your signature link over time. [I get quite a lot of visits as a result of my signature here on XF!!
]
Oh, and one final suggestion - get rid of the shoutbox!!! It takes up lots of your homepage and the dates on it are highlighting that there isn't much activity at your site!!
People want to get straight into reading a forum, so anything that pushes the forums
down the page isn't all that good!
Cheers,
Shaun