Portal 2's Peer Review not working
It took a good long time for to find this as well. Go into the co-op hub where you can see the big-screen display that shows things like steps taken. Stand in the center of the main platform and look at the board. The light bridge should be ramping down in front of you from right to left. Turn 90 degrees to the right; you should see the hanging surfaces that you portal into in order to fall on the bouncy plate. From here, you should be able to see into a hollow column on the right with a door and a couple buttons visible. Put a portal in and there and go!