if it's the one where you get one half from the statue, and the other from beating the sandcastle minigames, I'm pretty sure you need to talk to juno when she's standing out the front of the beach (when you click the beach, it's the first place you go to) I think morning/day, talk to her, and she'll take you to the Chest she found, You can then use both halves on the Chest and open it <3