Was it harassed by any of the other tank occupants? Does it show any signs of disease (rapid, heavy breathing, spots on gills, fins, or body?
Give it a couple of days to settle in. You should try live brine shrimp to get it started eating (not a good long-term diet, though), plus frozen mysis shrimp and spirulina-enhanced brine shrimp. Also, try attaching the nori/seaweed to a rock or something with a rubber band to see if it will eat that. You might put food in a cup of tank water and then use a turkey baster to blow it near the tang in case it is being out-competed or intimidated by the other fish. In addition, try soaking the fish in a garlic extract as that sometimes appeals to fish that are learning to eat new foods.
Was it eating at the lfs when you bought it?
It sounds like you put the fish in your DT without quarantining. One of the reasons for quarantine (in addition to avoiding introducing disease into the tank) is to let a fish settle in to captive live and start eating the foods you will feed without the complications of a bunch of other fish around.
I take it you have a big tank? The tang is likely to get quite a bit bigger.
