In Pokemon How do you find the key?

The Key Item can be found on Route 12 near Oldale Town in an item ball just right of where the Trainer battle with Cheren occurs in Pokemon: Black and White and Pokemon: Black 2 and White 2, or a bit past a gate north of Oldale Town in Pokemon: Diamond and Pearl, or next to an Old Chateau in Pokemon Platinum. After picking up the Key, the player can access Old Chateau which is previously impossible.