As a FedEx sales rep, I can tell you that finding the box might be hard. If you need something shipped, goto the AC. Most ACs have a pickup if they have a conference center there and will accept a package for shipment if the airbill is correctly filled out (I've done there quite a few times at the DFW ACs). It they won't accept it, they usually know exactly where the box is located.
According to our location guide, there is one in Term 4, and one at the hanger for AA. There is also a manned location at the FedEx cargo terminal that can take packages up to 9:00 pm.