Since it took me WAY too long to figure this out, I thought I'd post this for anyone trying to configure Cron Jobs on Media Temple's Grid Service.
First of all, you need to go to your Admin section and click on the Cron Tabs icon, then enter the following in your new Cron Job. (Change the orange text to your own server info/path):
/home/#####/domains/yourdomain.com/html/mt/tools/run-periodic-tasks
Then, open your run-periodic-tasks file (/mt/tools/) then go to line 12 and change it from:
use lib 'lib', '../lib';
to a direct path (and, again modify the orange to your info):
use lib 'lib', '/home/#####/domains/yourdomain.com/html/mt/lib';
(I'm on a Mac and opened it right inTransmit.) Hopefully my four-day search will save you some time.
Hopefully MediaTemple will include this in their documentation somewhere, as the tech support team wasn't helpful at all.
