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...
At Early Poptropica Island
If your talking about Counterfeit Island then you get the key and go back to the museum on Counterfeit Island
At the art museum in Early Poptropica.
You have to go to the early poptropica musem
When you get the message on Counterfeit island, travel to Early Poptropica in the blimp and go to the Pop Art Museum. The curator from the Museum Fantastique is at the lower left inside.
the curator is on early poptropica island in the pop art museum
On Counterfeit Island, the curator of the Museum Fantastique summons you to meet her on Early Poptropica (an extraordinary inclusion for Poptropica). Take the blimp to the other island, and enter the Pop Art Museum there. After meeting the curator, return to Counterfeit Island to continue the quest.
She is the curator of Museum Fantastique and she is in the Pop Art museum. Go to the lower left inside. (Note that she is only there where you travel to Early Poptropica as part of the quest on Counterfeit island.)
It is unique in Poptropica, but yes, that's what you do. Meet the curator of Museum Fantastique in the Pop Art Museum in Early Poptropica.
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 take it back to counterfeit Island and use it on the door of the house.
go to early poptropica and go into the art museum there you will find the person and then do what she said then go back to counterfeit island and there i hope i helped