1. Go to the Book Room which is above the Coffee Shop.
2. Open Captain Rockhopper’s Journal and turn to the last page.
3. There you will see the key.

The key can be used as a pin. It also lets you open Rockhopper's Ship’s Cargo Hold. Inside there is Rockhopper’s Notice Board and a Multiplayer game called Treasure Hunt. There is a book there to tell you how to play. It's a really quick and easy game to earn coins.