It's at DCA in a little area to the right of Monsters Inc. There is a little sign on a stand that indicates that it's for the Visa Meet and Greet. There is a CM standing outside that you show your card to.
Just wanted to let people know that I was able to redeem my ticket at the photo shop on Main Street. It was our last night there and I had forgotten to do it at DCA before they closed. I asked them and they said it was no problem.