**Spoilers**
Enter the mansion and take the flight of stairs. To your right, there should be a door leading to his bedroom. Once there, the wall to the right of the bed is where you want to go. Just slowly walk along that wall until you see the floating 'A' button that means to open. This will take you down a series of hallways, and at the end will be the secret room.
I got that problem too but I found out if you're doing the second quest with the brothers you have to kill the Banche then the door will open, hope this helps
go to the Pokemon mansion and go left and there is a door. go in the door and talk to the guy in the purple suit.hes the guy who owns the mansion.
NO!
It is next to the door to the mystery shack.
In the basement of the Pokemon mansion on cinnaber.
you dont
With a key my friend
behind the oakfield demon door.
footman
It is on the door on top of the knock sign
The key is in the Pokemon mansion.
A room full of money