This post is dated June 2018. I just got off the phone with AirBNB support without getting a solution to this problem. I could not find a "Switch to Host" option, nor could the agent. My problem is now fixed.
I have two google accounts set up with google, but only one set up as an account within the phone. I went into settings on my phone, chose the "accounts" tab, then added the second google account. Then on my AirBNB app, I logged out (Settings/Logout). When I used "google" to log in again, it gave me my two google account options. I logged into the google account option associated with the email address that airbnb was familiar with and wala, the problem was solved!
Now in the AirBNB app, I choose Profile, and am given a "Switch to Host" or "switch to traveling" option.