So to get the nightfall armor you have to watch halo nightfall and and after you go to Halo the nightfall armor will be available.So I watched Halo nightfall and it still won't give me the armor.
Odd. Unlocked instantly for me.
Here's what I would do:
  • Re-sign into Xbox Live and see if your armor is unlocked
  • Re-watch Nightfall and make sure you finish the video in its entirety each time
Often I've heard of people failing to get the Nightfall armor because they skipped like the last 10 seconds of the episode.
Anyway, good luck.