Go into the guest room - that's the room that you'll find if you go upstairs from the entryway, then when you reach Sam's room, take a 90 degree turn left and move straight ahead to the other corner. The door to the guest room is the one right in front of you, with a piece of paper sticking out from under it.
Go inside, and find the closet. Pull the lamp switch, and then look into the upper right corner by the closet door to find a switch. Click it, and one of the two empty bookshelves in the guest room will move to reveal a hidden passage. The achievement should now have unlocked!