I play Portal 2 on Xbox 360 and downloaded the Peer Review Dlc. I saved it to an external device (usb stick) where I keep alot of my game progress and data.
At the time I was about half way through the Coop mode and knew that I had to complete it to play the dlc. Later on, me and my friend finished the story (locally) and we were trying to find out how to get to the dlc.
I've seen that you need to get to the newest pillar in the hub, but when we looked at it it wasn't opening up so we couldn't portal in. It stayed closed and there is no visible way to get inside.
We've tried reloading the game and redownloading the Dlc but nothing has changed.