Unfortunately, in order to use this theme, you’ll either need a brand new WordPress install, separate WordPress install, or be willing to see your existing content disappear.

I'm probably in the minority but I actually like that idea. I've never been a fan of having a forum directly integrated with a normal site. I much prefer to keep them separate which is why I usually stick with SMF or phpBB etc. FluxxBB is a nice relatively lightweight alternative.