Atra du evarinya ono varda.
The Black Tower is of random set up - there is no definite map of it. Your best bet is to talk to people for clues on where the Key Trainer is, find the door and avoid trainers that do not fit the description given to you by the people you have defeated. There really isn't much to it.
Oh, and starting from level 6, the Key Trainer might not be on the same floor - you have to go to different floors in order to find him/her. It's much harder then the first five floors.