You'll want to click on the subdomains link to the left, click add and look through the domain drop down for one you like. Look under the 'Many many more available...' link for some good ones, like .uk.to or .us.to.
Once you've chosen, choose NS as the type, put ns1.x10hosting.com in the address box, and put your desired subdomain in the subdomain box. For example, for tnl2k7.uk.to, you'd choose .uk.to as the domain and put tnl2k7 as the subdomain.
Then you'd need to add your new domain (tnl2k7.uk.to for me) in the addon domains or parked domains in cPanel. Remember if you want the files to be in the public_html folder you should choose parked; addon if you want it to have a different folder. You could alternatively set it as your main domain via the account management panel (
http://x10hosting.com/login).
If you still need help, please make a new thread in one of the off topic forums and I'll help you there; as this one is for discussing the domain promo. Hope it works!
-Luke.