From this page you will be able to enter the name of the mailbox you want (the part before the @domain.com in the address) and also select the domain in the dropdown box, if you have more than one domain mapped to the package then they will appear in this section.
Click 'Add Email Account' and this will then create the mailbox for you with a random password and add it to the 'Modify Email Accounts' section where the password will be displayed for a brief period. You can reset the password from this section if you want a specific password to be used, please note when you come back to this page in future the password will not be displayed.
*** It will take a short period before the mailbox is fully active as the systems update so don't worry if this is not working straight away once created.***