I think you do this:
Login to your Cpanel (yoursite.x10hosting.com:2082 )
click on" Mail" which is the first icon
click on "Manage/Add/Remove Accounts" which is the third from the top
click "add account" at the botom of the page, to the left.
And btw if you havn't registered yet and you can't register it is probbly because X10 is not accepting any more applications, but you can try anyway just to make sure.