Your hosting account doesn't have that domain there. Either change the main domain through your account panel https://x10hosting.com/control/login.php, OR park it in your cPanel. Let us know when that's done first, and then we'll go over ftp details.
No, what he means is you need to tell the domain registrar (123-reg) to point the domain to x10. The nameservers are:
ns1.x10hosting.com
ns2.x10hosting.com
Then, you need to add the domain to an account as the main domain (done via account panel http://x10hosting.com/control), or park it in cPanel.