Ok, here's how I finally got it to work. On the Xbox One Store, I kept trying to install the Dark Below, but it would say I needed to pay. I finally remembered that I bought the Expansion Pack, which includes BOTH sets of Destiny DLC; The Dark Below and the yet-to-be-released DLC. So, I went back to the Xbox One Store and selected the Expansion Pack, not the individual DLC. I was then able to select "Install" instead of a payment button. It then installed just fine and I'm able to play the DLC on the Xbox One.