You cannot create an addon domain if your dns for the site you are trying to add hasn't prorogated yet.
To create an addon domain login to your cpanel and click "addon domains"
There are three fields cPanel asks for when creating an addon domain.
1. "New Domain Name:" You will put in the newdomain.com do not put www in the name!
2. "Username/directory/subdomain Name:" This will be the folder cPanel creates in your main accounts public_html folder. MAKE SURE that the name you give it doesn't already have a folder. Example... If you want to give it the username chris. You cannot have a folder in your account named chris already. If you do and still try creating the username for the addon domain to be chris you will corrupt multiple things on your account.
3. "Password:" fill in any password you want here.
