I don't know which control panel they use. It's a combination of different control panels. It doesn't really look that integrated as they show in their screenshots.
Actually, by date, I'm unable to login at the WebControl (that's how they call their admin), time-outs all the time. Yesterday the same happened, I thought it was a temporarily problem...
Anyway I've mailed them about my question about creating e-mail accounts on the fly, but they ignored my question. They referred to the KnowledgeBase where you can't just read how to create e-mail accounts manually.
I'm disappointed with MediaTemple. Everyone tells so many good stuff about them (I've actually mailed some others sites hosted by MT), but I'm experiencing problems from the first day I was activated.