You cannot create an addon domain if your DNS hasn't proprograted to the server already.
After you change the site's DNS you are trying to add, you will need to wait 24-48 hours before attempting to create your addon domain.
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 account's FTP. MAKE SURE that the name you give it doesn't already have a folder. Example... If you want to give it the username mdmed. You cannot have a folder in your account named mdmed already. This may cause unpredictable results.
3. "Password:" fill in any password you want here.
