After logging into your control panel, go to Domains > Addon Domains, after which you will see these options.
New Domain Name: Enter your domain you wish to add here.
Username/directory/subdomain Name: When you click outside the New Domain Name field, this will be filled out for you. You can leave it this way, but if a name you already used is suggested, you will need to change it.
Document Root path: When you click outside the New Domain Name field, this will be filled out for you. You can leave it this way, but if a path you already used is suggested, you will need to change it.
Password: Enter the password you wish to use for this domain.
Now click the Add Domain button to finish.