why do you want it to redirect, go to the website you bought the domain from and set your nameservers to ns1.x10hosting.com and ns2.x10hosting.com then go to your cpanel and click on addon domains i think (its been a while since ive had hosting and no1 seems to be helping me here) and then add your domain, make sure you dont get both your subdomain and your domain indexed by google or else one of them (probably your domain) will get banned from google.
Another option would just be setting the nameservers and wait for an admin to change your account from that subdomain to your domain so that way you wont have to worry about them both getting indexed.
or create a robots.txt, anyway im rambling now ill shut up. lol
Log into the account you were given for the name at the domain registrar, then from there you should be able to set the nameservers to ns1.x10hosting.com and ns2.x10hosting.com, like oab said. Then, if you haven't already, link your cPanel account to your forum account (see this page). From there you should be able to change your subdomain to the Top-Level Domain that you purchased.
Regardless, it is now mandatory that you have your cPanel and forum accounts linked.
Edit: Lol, uh, wow. It works now. What I did was, I made an addon domain redirecting mydomain.x10hosting.com to itself (mydomain.x10hosting.com). I then deleted the redirection, and everything was fine. Very strange...