To the right of the table is a genealogy chart on the wall, below which is a coat of arms.  Press the three roses in the following order, left, right and bottom.   You will hear a 'clunk' to your right. 

A secret catch has been released on the portrait to your right.   Go to the portrait and click on the catch that should be sticking out of the side.

The portrait will swing back to reveal a safe.  Turn the knobs to match the coats of arms of the Regents shown against it (you may have to go back to the book on the table (reveal)).

Insert the brass coloured key and open the safe, take all the keys in the safe, including the red coloured one and the piece of paper.

Leave the safe and go through the door opposite the one you entered by.