To see all your current domains sign into the control panel and then click on the "Account Manager" icon, followed by the "Domain Setup" link.
You can see all your listed domains from here. Also, you can add new domain or rename existing one. To host an additional domain, click the "Add New" button at the top of the page.
First, enter the domain name. Use all lower-case characters. You do not need to enter "http://" or "www."
Second, select the bandwidth and disk usage available to this domain. If you only want 1 gig of transfer per month, enter 1000 into the "Bandwidth" field. If you want to share bandwidth with your main domain, without limit, then check the "Same as Main Account" box. Finally, click "Create."
You can also choose what access you website will have. Click only if you know what you are doing, otherwise just leave it default.
Make sure to edit the record of the new domain so that it uses the same name servers as your main domain (this is done through the domain registrar). It may take up to 72 hours for the new domain to resolve.