So let me start by saying that I've read and completely done all that I think is articulated in this great post between Andrea-r and a few others: http://wordpress.org/support/topic/domain-mapping-how-to-video-help
On the second page of that discussion, the original poster defines all the steps she took and that it all worked. Oh how envious I am.
I too have my sites hosted at MediaTemple.
I purchased the domain at MT in case that was easier than having GoDaddy do this particular registration for what I wanted.
I started by connecting it up to the hosting account I had. That creates all sorts of entries into their DNS ZONE file, which I want (in order to manage email, etc).
Then I deleted the entire website folder for that domain (xs-lounge.com) and created the symlink using the article referenced. It worked. I tested by reviewing FTP and the symlink is in place.
So then I test the site before doing the domain mapping. I created the site at xslounge.wpadvisor.com and it loads up just fine. In my ZONE file, I have an A record for *.xs-lounge.com that points to the IP that is for my main account (wpadvisor.com) and I think it's a fixed IP because I also bought an SSL cert and they had to do it (for my ss account).
When that was all fixed, I then went to map the domain. I didn't pick primary because wpAdvisor.com is the primary. But I put in xs-lounge.com (not http://www.xs-lounge.com).
When I go to http://xs-lounge.com I get a blank screen.
But when I go to http://www.xs-lounge.com I get routed to xslounge.wpadvisor.com (which I really don't want to see).
So I tried adding a cname for http://www.xs-lounge.com in the ZONE file that pointed to xslounge.wpadvisor.com but that still didn't do it.
Anyone have any clue what to try next?