Once a domain is created you will see it listed under Domain Management. Clicking on the domain name itself will allow you to edit its attributes and add users to this domain.
To add a user, give the user an e-mail username in the E-Mail Username field and fill in the real user's e-mail address in the Recipient field. Click the Add New button to add this user's e-mail.
Additionally, towards the bottom of this menu, the current configuration can be changed from here.