Okay - gotcha.

Yes, ideally you would host your own blog & forums, instead of using free hosted options. But you can also create a redirect from the domain since it's hosted at HostGator.

So to make motocrosshideout.com/forums point to your free hosted forum, you create a redirect file and name it index.html or index.php and put it in a directory named "forums" on that account. Then when someone clicks that link it redirects them to the forum you have on the other site.

Honestly, I would go ahead and set up the blog & forum on your own hosting account before you get too much further with the project. You need to own the properties and the content, especially if it goes big.