How'd you miss it? 
As Bunyan said its on the Singleplayer menu. Oddly it seems to include dev commentary for all of Co-op too. You can't save (at all) while doing it though which is a bit of a pain, so you're better off giving yourself the time to play through entire chapters at a time.

As Bunyan said its on the Singleplayer menu. Oddly it seems to include dev commentary for all of Co-op too. You can't save (at all) while doing it though which is a bit of a pain, so you're better off giving yourself the time to play through entire chapters at a time.