Lobsters are scavengers and will eat what ever they find. I would feed it a piece of shrimp or clam every once in a while. They should eat pellet and what ever it finds on the bottom.
I would skip the lobster, they can and will eat whatever they can get their hands on. I had one in a freswater tank that attempted to decimate my fish population.
Most likley what you see at the LFS is a blue crayfish. The store in my area keep them in plastic boxes in a tank with fish. They will eat anything they can get a hold on, including your fish and each other.