Once you figure out where Carly is, you must arrive at that same T-Mobile location and check in using the Foursquare app on your smartphone. Once you've done that, you need to find the contest judge who will be at the store with Carly, we assume.
The winner will apparently receive unlimited data for life. Not a bad prize to win. And if you're lucky, who knows? You might get to chat up the carrier's now iconic spokeswoman for a few minutes.
source: T-Mobile via TmoNews