You must have your domain name's NAMESERVERS pointing to ns1.ethicalwebservices.com and ns2.ethicalwebservices.com before you can add an addon domain.
You cannot create an addon domain if your DNS has not propagated to the server already. So, after you change the sites dns you are trying to add you will need to wait 12-24 hours before attempting to create your addon domain. Now goto the control panel, 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 FTP. 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 run into a lot of trouble.
3. "Password:" fill in any password you want here. you don't have to use this password but you can if you want to login directly to the addon domains website fron your FTP client or if you have others working on that particular site with you, you can given them this login information so they only have access to that site.