If you want to find this area, see my guide for the following trophy:
Once you're in the hidden room, you'll walk through some small hallways to a large puzzle room. Just before you get to the room, you'll walk across a slope, with a green ball at the bottom and a green ball entry point at the top. Push the reset button at the top to have a new green ball drop down.
This is something you haven't done in the game so far: touch the ball yourself. With
, it is possible to give the ball a push. Make sure it goes on the slope and ends where the other green ball was when you entered, and the trophy is yours.