Log in to Control Panel
You can find a guide for that by clicking this here.
Open Domain Setup
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.
Click Add New
From there, look to the right side, and find the ADD NEW button.
Fill in Information
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.