Please don't bump (and remember that we're all volunteers here, and we help when and if we can).
I've installed the domain mapping plug in but that's just telling me to use my cPanel that I don't have
Assuming this plugin: http://wordpress.org/extend/plugins/wordpress-mu-domain-mapping/
The reason for that is you need a couple things setup on your site do the redirect.
And if you read it carefully it says this:
If you are using CPanel, use the Park a Domain menu to set the mapped domain to your main installtion.
So you're not and you can skip that part.
What you need to do is this part:
Make sure the default Apache virtual host points at your WordPress MU site or WordPress 3.0 network so it will handle unknown domains correctly. On some hosts you may be required to get a dedicated IP address. A quick check: in a web broswer, type in the IP address of your install.
You may want to read this: http://ottopress.com/2010/wordpress-3-0-multisite-domain-mapping-tutorial/ which is a very nice tutorial on how to setup domain mapping.
As otto mentions
Every host is different, and you’ll have to talk to your host to make them able to point the domain name at your existing site. How to do this varies from host to host, but the important thing is that when you visit your new domain (before you do this!) then you want it to go to your main site, as is.
For GoDaddy, you may want to read this post: http://wordpress.org/support/topic/wordpress-30-and-domain-mapping