Log into Cloud Control
From the System section, click Domains
Click on Add Domain
Choose whether you're adding a Domain or a Sub-domain. In most cases, you will choose Domain
Enter the name of the domain in the Name field.
Click Add Domain or click Cancel to disregard all changes.
Now scroll to the Hosted Organization section and click Domains
Click Add New Domain, select the domain to be added as an Accepted Domain for Exchange and click Add Domain
Your domain will now be available to use against all objects in your Exchange configuration.