there is no such thing as a freshwater crab. all crabs need salty or brackish water to survive. if your petstore told you that they were freshwater, it is only to market them more easily because few people have brackish tanks. put in a teaspoon and a half of of salt per gallon of water in your tank.
someone please correct me on this. I have NEVER seen a freshwater crab in a petstore, only fiddler crabs and red thai crabs, which are both brackish (as wel as several other brackish crabs). if there are other easily aquarium-compatible freshwater crabs, please tell me.
as for feeding, try some peppers, cucumber, or zuccini. make sure its fresh though! and celery, too, sometimes they have fun tearing it apart by the xylem.