Moving one of my cPanel hosted domains to WPMU mainsite.com

I am trying to use the Domain Mapping plugin on a WP MU install from WPMU.org
When I try to park mynewsite.com to mynewsite.mainsites.com, I am getting the following error:

Error from park wrapper: mynewsite.com is already configured. Sorry, that domain is already setup (remove it from httpd.conf)

Do any of can I find information on how to fix this?