This one is the most involved out of all of them.
I don't remember exactly how it works, but this is how it goes according to memory.
First, you will need to select "Haunted Hallway" from your tablet menu and go to it. Once loaded, (it will be really dark which is a little bit aggravating because you have to strain your eyes to see) but anyway, once loaded, you need to advance to the second floor by finding the key which is in one of the rooms on this floor. (it's not hidden, it's out in the open) Once you have that, go to the writing on the wall that says "Find the Key" and you can unlock the door.
Now, you need to go into every single room in this floor and go over by the bookcases to your left and get everything to "trigger". Which just entails walking near everything. So just go into each of the rooms, and walk around a little bit in all of them. Also go over to the bookshelves to the left of the entrance. *There WILL be a clown pop up and scream outside of the doorway of the party room. This is NOT the correct clown. Once you've triggered everything, go back into each room again and in one of them, the door will lock and you will have to watch a clown do something for about 30 seconds, then you will get the achievement.
Short version: find the key on the first floor in one of the rooms.
Then on the second floor, look through everything TWICE.