I'm having this problem, too...
1. Pre-ordered the game from Microsoft store (sweet, can't wait)
2. Got the game, downloaded and installed Halo (that didn't take so long...)
3. Found the Halo Channel app separately, installed it (ooo, shiny)
4. Attempted to watch Nightfall from the Halo Channel app, it tells me to buy it, then sends me to the game (uh, what?),
5. Click the link in the game, it sends me back to Halo Channel (huh, helpful?),
6. Repeat steps 4-5 a few times (just to make sure the buttons on my controller work),
7. Realize I have a code in the game box, use it (no way this can go wrong)
8. Code approved, automatically attempts to download Halo Channel (uh, already have it, so...)
9. Halo Channel download success, downloads Nightfall (win!)
10. Repeat step 4, which leads, inevitably, to step 5 (controller still works, btw)
11. Uninstall Halo Channel (running out of commentary...)
12. Attempt to re-use the code to download (fail, already redeemed, see step 8)
13. Re-download Halo Channel, re-download Nightfall (deja vu)
13. Repeat step 4 -> step 5 (more deja vu)
At this point, I realize I probably should've just used the code first and never downloaded the app separately, but that's not an option anymore. Thoughts?