From what i remember of this part of the game you need a piece of card and a thin piece of metal to poke through the lock.
First put the piece of card under the door, then using the piece of metal push the key from the out side of the lock and dislodge the key this should then let you open the door from the inside.
I too have reached this point, however when I try to use either the cardboard or the metal pin, nothing happens. I've tried dozens of times. I've even switched off the console and re-loaded the game several times, but still nothing happens when I try to use either item.
Help please!
Nick