go into the master bedroom. Next to the bathroom door their is a closet. Open it to find a safe. To open the safe you need paper and a pencil and use it with a book on the mirror desk in the room. It contains the code to it and you will be able to access the diary.
madisons diary is in the safe to get the code you will need to get the pencil and paper and place it on the book in the master bedroom, make shore to close the closet door so you dont get caught