Log into the ControlPanel at http://yourdomainname.com/ControlPanel

The Config Emails link will give you the username and password.

Click "Add End User" in the sidebar

Fill in the information.
   Be sure to remember the password
   Email accounts typically have 1000mb to allocate amoung all users

Click "Next"

Note that the mailbox name and the email address are not quite the same, the mailbix name will have a prefix.

Click "Next"

Click "Save".
  The new email address is instantly ready for use.