SPOILER!!!
Spook - change the floor so the rat head thing pops into the other room. Take the tiles off the wall in the other room and put them in the slots underneath the stairs. Catch the rat head thing in the diskette box and put it behind the tiles in the stairs. (you need to pull the tiles back a bit) Door should open then.
The plant doesn't do anything, but you might need the chair to reach the button on the top of the stairs.