First of all make sure you get some salt in your set-up >> they're brackish to marine those crabs and need aspecific gravity of between 1.005 and 1.010 (you're gonna need a hydrometer). As for feeding - there are crab-specific foods, but the freeze-dried plankton will do, as well as F-D shrimp and sinking food pellets - they're essentially scavengers in the wild...
PS - make sure you've got some dry land for them to come up on to