WARNING: This guide will be spoiler heavy, proceed only if stuck.
For the DLC you beed to open the safe. To do so, you have to find clues to get it open.
But first you need the master key to get in to the West Wing of the house.
You can beat the game to the punch, turn around and head in to the entrance way. To the left of the front door is an document "Letter to Jonah" and once facing it, you can turn around and open your suitcase for a flashlight.
Head back to where you came from and search around the room with the safe. Pick up everything. But what you're concerned with most is the map not far from the locked library door, on top of some boxes.
Now use your Instincts to find the hidden passway in the same corridor. Follow this corridor along and find the secret entrance to the library.
There are a ton of collectibles in here, so have a quick explore round before continuing.
The one you REALLY want is right near the entrace you came in, a blank map. This is entitled "Blank expedition map".
Find the lighter in the library turn around and light the fireplace behind you. head over to the other side and move the ladder along the backwall, climb up, move the shelving to your right towards you and climb down. Move the ladder back to wence it came and climb again, follow the passway along to find a document and a locked chest.
Head back to the fireplace and use the map on it. Under heat it gives you a clue to the mystery.
There is a stand right behind you by the bookself, that now flashes through using instinct. Move it to find the Master Key underneath.