The Guest Relations offices at DTD are located in the Marketplace and in West Side.
The Marketplace location is right next door to the Once Upon A Toy shop on the Rainforest Cafe end of the Marketplace. This is an actual office that you walk into. It is open until 11 pm Sun-Thu and until 11:30 pm Fri & Sat.
The West Side location is at the Pleasure Island end, just past the AMC Theaters. Walk straight down with PI on your right and AMC Theaters on your left and you will see it right in front of you. It is a white building that says "information" on it. This is a Guest Relations window like the parks have. This one is open until 11 pm.