To view and edit a users email aliases, first select the user from your users list. Next, click on "Aliases"

Aliases that have been added previously will be listed under User's Aliases. These aliases can be edited or removed. 

Domain Aliases for the user will be listed under Non Editable Aliases. These aliases can not be edited or removed. 

Click on the box Alias' Username to enter the username of the alias. Click on the second box to enter the domain. Click on the Add icon to add the alias to the user.