You could probably use a sprayer filled with saltwater to really moisten the rock and put damp paper towels on it if you absolutely have to wait until Friday to put the rock in a tank.
Like others I say that you should put the rock in as soon as you get it; this lessens the die-off considerably. Why would you pay for live rock if you let iti all die? It will already be out of the water for at least a day (if not more) so the organisms on it need the water to live. I believe that medium-sized crabs can live outside of water for 14-30 hours before they really start to decline. Other animals is less, I'm sure.
You know that you don't have to aquascape anything yet, right? You could have your tank filled Wednesday with sand, put first rock in Thursday, then aquascape Friday. Doing it all at once can be aggrivating.