Instructions
Proceed through the game until you've sculpted the statue. At his point, a pearl appears in the sand.
Drag and drop a character with the Master Building skill onto the pearl. He'll take it over to the waterfall to clean it off. When he's done with it, he'll drop it there.
Pick up a Master Scientist and drop him on top of the cleaned pearl. He'll polish it and then drop it.
Move the village chief onto the pearl. She'll feed it to the giant clam, getting a key in return.
Drag and drop the chief onto the mystery door and she'll put the key in it.