Here is how to unlock the Hidden Key by playing Puffle Rescue:
1. Use your map and go to the Mine Shack
2. Walk to the Mine
3. Play Puffle Rescue
4. Select the Black Puffle Mission
5. Get the black puffle by going a little upwards in the water.
6. Follow the squid and it will lead you to the Hidden Lake. Be sure to go above bubble’s to save yourself from drowning.
7. Once you have finished the level it will lead you into the Hidden Lake. You are now able to pick up the Key: