You can find a guide for that by clicking this link.
Once logged into the control panel, you can add as many domains as you need. On the left side, click on Account Manager, and then Domain Setup.
From there, look to the right side, and find the ADD NEW button.
After clicking ADD NEW, you will see this page:
Fill in the Domain field with the one that you wish to add. Ignore all of the other fields on this page. When finished, click CREATE at the bottom right.