From the control panel, click the "Mail" tab and select "mail accounts"
Click "create email address"
Type in the email you want to be @yourdomain.com
You can add an external email address that can be used to recover the password if you forget (you can also reset the password from your control panel).
Type in or generate a password, confirm the password by typing again.
Choose to limit the email space or leave it unlimited.
Write an optional description (useful if you have additional users).
Click "Ok"
You can login to your new email by going to webmail.yourdomain.com