Active Forums is a free open source project supported at
http://activeforums.codeplex.com. There is probably a bit of confusion here. Active Forums was a commercial module, was purchased by DotNetNuke.com, and has transitioned to a free, open-source module. All "core" modules are free. Active Forums is free.
There is also an obsolete Forum module that was retired after Active Forums became open source.
On problem that you may have is that Active Forums requires (I think) DotNetNuke 6 or 7. So, if you don't want to upgrade your site first, you will have to use the obsolete Forum module. If and when you do upgrade, you can also migrate Forum to Active Forums.