Ok you'll need that. Assuming your in that room with the rainbow dots one the wall, where titania tells you to split up, go through the door at the top of the room. Jump onto the platform in the middle of the room and move the garbage so that you can jump onto the platform on the left side, and raise the water. Go through that door and walk up and all the way left and enter the door at the end of the platform. Make your way through that room to get to the table in the lower left corner and grab the key.
I tried to write that as clearly as I could, if you're still confused, look at this map in the spoilier. The room you want is top row, second from the left.
http://www.pokemonreborn.com/forum/index.php?showtopic=10938&page=18