You don't own that domain (web address). You need to purchase the domain from a registrar before you can set it up in cPanel. Alternatively, you may login to your account panel (http://x10hosting.com/login) with your forum username and password. Once logged in, look where it says 'Account Details' for the domain box. Click the arrow to the right of it and select .x10hosting.com or another option that you like, then type in what you want before it on the box to the left. Now click the Confirm Hosting Details button.
for example, for user.x10hosting.com, type 'user' in the box on the left, and choose'.x10hosting.com' from the drop down on the right.
Agreed with tnl2k7. If you have any problems doing so, tell me what you want, NAME.x10hosting.com, name.exofire.net and i'll go in and set it up if you can't.