This is how it worked for me, and it should work for most everyone, if not everyone else: Go to the Halo Channel and select Halo: Nightfall. Once you see it, select View Episodes. Then hit the fifth and final Episode and let it load for a moment. Either fast forward through it or skip it, and go to your Rewards. The Halo Nightfall armor set is the last one (eighth reward of the whole series) and press A on play Halo. That will take you to the Halo 5 Guardians Beta Download screen. Then download it. 