Hello, So my boyfriend and I were having the same problem; not being able to access the either Nightfall or the Terminals. We downloaded the game like normal and uploaded all the codes required for the Channel and the Skull. We then jumped right into co-op campaign. With BOTH of us playing we were NOT able to access the Nightfall Video or the terminals. After reading everything I could on ALL the Forums here and getting no answers we tried it on JUST SINGLE PLAYER and it worked!!
Play Pillar of Autumn on SINGLE player and you can access the first terminal right after speaking with Keys.
Play ALL the way threw that level, save game and go to Extras, where you will click on Nightfall.
It was able to play for us right after this. I hope that helps and if it does pass on the word so everyone can watch and have fun 