Few years back I Also run this same form. I added lots of fresh content on that. But google not index my forum. I still not under stand why this happen to my forum. I did directory submission blog commenting and forum posting. but nothing happen with my forum
Getting a forum urls indexed in search engines is tough but not impossible. Forums are dynamic sites so they needs some serious hard work to get indexed. Your content should have be totally unique so that can go viral or you must have plenty of quality links to your site to get indexed. Here is a topic discussed about forum SEO, take a look a at it: <!-- l --><a class="postlink-local" href="http://forums.seomotionz.com/viewtopic.php?f=60&t=705">viewtopic.php?f=60&t=705</a><!-- l -->
A lots of quality links can help you get indexed. But the most important thing that matters is content your content has to be totally out of the box to get indexed faster.
Link building and developing good content is not enough for a forum site to get indexed. You must also get good outreach for your content. The more the content of yours will be sharable the more it can get indexed faster.
Use social bookmarking sites not to gain links but for outreach. Try to reach as much as crowd as possible. Engage them when they visit your forum. Forums are for people so if they like it the search engines will like it too.