If you've registered a regular ".com" domain, you can always put in one of three places:
1. main domain - you can change this setting in your account panel
https://x10hosting.com/control/login.php, just click on hosting details.
2. addon domain - you can add/change this in your cPanel under Addon Domains
3. parked domain - you can add/change this in your cPanel under Parked Domain
Make sure that you go to your domain account at your domain registrar and add the following primary and secondary nameservers:
ns1.x10hosting.com
ns2.x10hosting.com
If you haven't registered a domain, find a domain registrar and purchase one there.