**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.
With a key my friend
behind the oakfield demon door.
In the basement of the Pokemon mansion on cinnaber.
you dont
kick a chickin in it's mouth
By opening all of the others.
You get in the demon door behind the Bordello in the Dark woods. To get through the door you have to sleep with 10 people.
A room full of money