Domain Mapping to same ip in Media Temple

Hello, I’ve been working on getting a multisite install up and running on wordpress 3.3. It seems to be working fine if only I could figure out how to set up the domain mapping.

Here is how I have things setup:
I have a subdirectory install so I have domainA.com/domainB. domainA.com is where I have the multisite install and domainB.com is the site I want to map to. Both are pointing to the same ip as they are hosted on the same server a Media Temple(DV).

As both of these share the same ip I’m not sure how I need to set up the DNS. I’ve read that you can accomplish this by editing or adding a CNAME record, but I’m not sure what edit I would need to make.

Any insight as to what I could try next would be greatly appreciated.