Previous instructions are incomplete. Try this instead:
1. Open the Airbnb app
2. Go to Trips (bottom tab bar, middle)
3. In a list of Upcoming Plans, select the one you want to add to Wallet by tapping anywhere on that big square
4. Where it says Where you’re staying, tap on Your apartment reservation (but NOT on either Contact host or Change or cancel...)
5. In the details for the apartment (or house) reservation, scroll ALL The way down, to the end of the details (it’s quite a ways down)
6. At the very bottom, you’ll now see Add to wallet >, tap that
7. Add the pass you’ll see loaded by hitting the Add link in the upper right-hand corner.
DONE.