If you've purchased the domain, and want it to be your primary one, update the domain information in your Account Management Panel (
http://x10hosting.com/account ) and than create either a new add-on or parked domain in your cPanel, which reflects the information of your domain.
You will also have to point your domain to the x10Hosting Name Servers (ns1.x10hosting.com & ns2.x10hosting.com).