To add the addon domain:
Please make sure the name servers are changed with your registrar to the nameservers assigned to you. They would be the same as the primary domain is using. They can be changed before or after adding the domain.
To add this through your cPanel. Choose the menu option "Add-On Domains"
New Domain Name: the name of the domain (ex: yourdomain.com)
Username/directory/subdomain: the FOLDER you wish for the domain to point towards (ex: yourdomain)
Password: password for your account (*recommend same as cPanel)