Hello CWheat,
Actually your newdomain.com is not "Parked" it is added as an "addon" domain and hence it is pulling the index page of its own. You do not require to modify the zones for adding the parked domain, just delete the zone that you have added and then using the "cpanel" add the newdomain.com as parked domain, cpanel will atuomatically create the zone and .db file for the newdomain.com and add the "virtualblock" entery for the newdomain.com to point to the public_html folder of your "mymaindomain.com" that's it.
If this is confusing to you and if you have the root access to the server then open httpd.conf file find the "newdomain.com" and set the 'DocumentRoot' value as /home/yourusername/public_html (where yourusername is the username of mymaindomain.com) and then restart the webserver. Your newdomain.com will now fetch the pages from mymaindomain.com.