In the mansion, you go to the two or three black holes in the ground. After you battle the scientist dude he will say "If you get stuck, try jumping into one of these holes." Jump into the first one. You will be in the basement. Go all around opening and closing doors and in the back room you will find the key. (You can also get ditto down there!)
On Cinnabar Island
There is no Pokemon Mansion, it was destroyed by the Volcanic Eruption in Cinnabar.
Cinnabar mansion
seafoam islands, surf to the right of cinnabar. to find a walkthrough for the caves of the islands, try googleing them
Somewhere in the pokemon mansion is a key that opens up the cinnabar gym. You need to beat some trainers and then you can battle Blaine. Hope that helps.
You have to use surf off the right side of the island to get to the seafoam islands
its in seafoam island not cinnabar
its in the mewtwo mansion in the basement
The Pokemon Mansion on Cinnibar Island, and after you beat the Elite Four, Mt. Ember of the Sevvii Islands.
Cinnabar
If we are both on the same page, and you and I are both thinking about the same Pokemon masion inleaf green version..... the masion is found on Cinnabar Island. One easy way you can get to Cinaabar Island is by flying back to your hometown (Pallet Town) and "surfing" across the little path of water at the very south of the town. Once your on the water, keep moving south and you'll eventually get to Cinnabar Island, home of the Pokemon mansion! (:
Cinnabar Island