X10Hosting gives you a free subdomain. Something like <name>.x10.mx. You are not required to add another domain to have your website up. You already have the one when you signed up. Domains are usually bought. If you want free, there is Dot.tk.
But if you have a domain and wish to add it to X10, login to cPanel. Go to Domains > Addon Domains. Make sure the domain is pointed to X10's DNS servers before continuing. Fill up the textboxes.