Go back to Counterfeit Island and use it to open the Inspector's house in the Country Side. On the way, the street artist gives you a clue as to what to look for in the house.
If your talking about Counterfeit Island then you get the key and go back to the museum on Counterfeit Island
Early Poptropica.
The key is not on counterfeit island. Go into the mueseum and the person on the left will give you a package. then go to early poptropica and there will be a lady standing on the left.
The art thief is back on Counterfeit Island. As part of Counterfeit island, you don't need to do anything in Early Poptropica except get the key from the curator at the Pop Art Museum, then return to Counterfeit Island to open the Inspector's house.
You now have a key that fits a lock on Counterfeit Island, and you haven't found a key to the lock on the Inspector's house, so...
you go to early poptropica, and go inside the gray art place. there you talk to the lady in purple and get a key. the key goes to the locked house back in counterfeit island.
The old woman is the curator of the museum on Counterfeit Island, and she came to Early Poptropica to recover a key you will need to complete the quest. When you receive her message, go there, get the key, then return to Counterfeit Island to use it at the Inspector's house in the Country Side.
Once you get the key from the curator, go back to Counterfeit island and use it at the Inspector's house.
You take it back to counterfeit Island and use it on the door of the house.
She warns you that there is a notorious thief at work on Counterfeit Island, and gives you the key to the Inspector's house in the Country Side.
You go to Early Poptropica Island and go into Pop Art Museum and a lady gives you a key
Since you couldn't find a key to unlock the cottage in the Country Side, this is probably it.