I have the key, I just have been trying to get it open and I have no idea why it won't open.
You must put both the Key and the Locked box in the same room of your house. Then Save the room and leave. Then go somewhere, like Downtown, and then go back to your house by pressing "MY PLACE".
Go to the room you left them in, and click on the key, you should get the blue box that opens when you click an item in your house, that says things like Raise, Lower, Remove....ect. ect.
A new option will appear when you click the key......"TRY". Press "Try" and a pop-up will appear saying you opened the Box, and it will give you some millsbucks. Thats it. Once you open it you can't close it again. And you will now have an "OPened BoX".