Don't look for it in the tank, it will come out in time and stressing it out will just make it harder to acclimate. I have collected both of the common Atlantic species and when you first put them in they will dig into the sand and stay there.
They will eat prepared foods eventually but Klepto is correct, they are not easy to feed. The food has to be the correct size and has to be moving across the tank about 2 to 6 inches off the bottom. They are also very shy, at first every time something moves in front of the tank they will go down into the burrow, so to watch them you have to be very still with the room dark and the tank light on. This makes them even more difficult to feed because the eel is going to drop under the sand as soon as you approach the tank, and then you have to make sure the food is moving around the tank when he finally comes back out, the other fish can't have eaten it all and the filters have to keep it moving and not suck it up, not an easy combination to over come.
What I did (and what most people who are successful at keeping them do) is to keep them alone. They are social so you can keep several together but any aggressive feeder or active fish will just disturb them. I had 3 in a 60 gallon cube and put a mirror on the back and sides of the tank along with live eel grass. The mirrors not only made the tank look bigger but it also gave the eels the feeling that there were more of them in the colony.
