Once logged in to the control panel, make sure that you have the domain selected in the top-right:
Once you have the domain selected, click on E-mail Manager and then E-mail Accounts on the left navigation.
From there you see a list of email accounts. Note that your Control Panel Username always exists as an email account on the domain. This is necessary for back-end functionality, you cannot delete it.
From there, click on CREATE ACCOUNT.
From there, you will see this interface:
Email Quota is the amount of disk space in MB that you want that account to be able to use. Send Limit is a daily limit on outbound email, which goes to a max of 7200 (or 300 per hour, as is our limit).
Finally, when ready, click CREATE ACCOUNT at the bottom right.