Login to your Control Panel at:
On your Home Page select the tab Hosted Domains.
On the next screen select Add Existing Domain Registered Elsewhere.
In the Domain field enter the domain name (without www).
Zone Management ability; check that box if you are going to immediately manage the domain's DNS records, otherwise add that later.
Lastly select the subscription that you will be using the domain with, in the case of Web Hosting that is most likely Website (Apache) and in the case of Mail Hosting that is most likely Microsoft Exchange.
It is OK to if both Web Hosting and Mail Hosting are selected.
It is also OK to select multiple domains to either a website hosting subscription or an email hosting subscription.