You'll reach the point in the DLC where you learn to use the flashlight to kill the "darkness" enemies. You'll then pick up a key and open the door on the top floor, to the left. Once you do this, go back to the library (middle floor, right-hand side). You'll want to pick up the grey book in the corner. Now, go to the main floor, and proceed all the way to the left. Put the book in the bookcase to open the secret door, and go inside to push the urn off the table.