MyBB looks good. They generally have nice clean simple skins and this is one of them. Only thing is though it's still a bit out of the box and default looking. The logo is great but you need to do other things also to customize and personalize it. Start with the forum icons.
You've lots of little minor things that need to be done. Here's a few
Remove the calendar. It's pointless and Google get's lost in it.
You have "CONTENT HIDDEN! PLEASE JOIN TO VIEW THE HIDDEN CONTENT!"
Edit that. Maybe add a nice image. Not an ugly heavy one that will slow the site. What every you do their must be a direct link to register to make it easier to register.
You could do with a register banner. Again not an ungly heavy one. Something simple and light that fits the skin but doesn't distract too much.
Your probably going to have to do a few link exchanges. Do them with similar sites. CrackingForum and other cracking sites, maybe even blackmarket sites. Try to avoid link exchanges with crappy warez sites who's traffic rely on DDL sites.
For SEO you have limited content and most likely never will have thousands of topics as it will be more community based unlike warez sites with thousands of automated topics.
Therefore you'll have to make sure what when Google does visit your site it sees what you want it to see. Add a nofollow to the new register link your adding to the hidden conten. That way you won't have Google going to pages their is no point indexing. Also add nofollow to stuff like VIP, Members List, Register etc in the navbar. Then go into the site and look at every single link on the site and ask yourself does Google need to follow this. eg New Topic links, Reply links, Search, Register etc etc.
Read my SEO Tutorial (link in my sig) for more tips. Best of luck with it.