You do not have to find the key. It is given to you later in the game.
You use the key from the Curator to unlock the Inspector's house in the CountrySide.
First go to skullduggery island,take your ship to a island called Keys World island and you will find the key of the chest on counterfeit island!;)
When you finally receive the key to the Inspector's house (from the Museum curator, who is in Early Poptropica), you may have some idea who the Black Widow is, especially with the name "Veuve Noir" on the sign.
Since you couldn't find a key to unlock the cottage in the Country Side, this is probably it.
The key to the investigator's chest on Counterfeit Island is hidden in the bushes to the right of the cottage. You need to use a stick to reach it and then unlock the investigator's chest to get the next clue.
After you finish the scooter chase and get the key from the curator, enter the Inspector's house in the Country Side. There is a new paiting on the second floor, and one corner is loose from the frame. Peel it back to find the stolen painting "The Scream".
Around the island, there are pieces of paper. Find all of them and piece them together to find out how to unlock the lock. Good Luck!
Check the painting its pealed in the top right hand corner click it and see what happens next...
After you unlock the house you go in and then go to the second floor, as soon as you get there theres a painting. on the top right corner of the painting there a piece of paper stickin out. click on it and then you done.
Once you pass the test by the assistant curator, and complete the Forgery Training, you are hired and receive a key to the Supply Room.
You have no way of going to the Supply Room until you are given the key by the assistant curator. (Complete the Forgery Training.)
If you are referring to the keycard in Poptropica Counterfeit Island, you use it to unlock a door in the secret lair, which can be found in the countryside. It is locked so you have to find a key.