Click on "Business Email Hosting - YourDomain.com" in the list of Your Active Products/Services - if you don't have email hosting with us yet, you can purchase it here: https://www.domainnamesanity.com/cheap-email-hosting.php
Click the "Setup Mailbox" button in the left hand sidebar. If you have already used up all of the mailboxes you've purchased, you'll be prompted to upgrade.
Type the new user's Mailbox Name (username) and password in the Password and Confirm Password boxes, and click Save. The "User Info" fields on the right hand side are optional.