1. I have ED on steam, where I purchased the alpha deluxe version.
2. When I click play on steam, it gives me the option for either Elite Dangerous, Elite Dangerous: Horizons, or Elite Dangerous VR:. You must select Elite Dangerous
3. When the launcher comes up, you are given the same options you had in steam, plus an arena option if you just want to play CQC (not what we're talking about, but saying so for clarity)
4. In those options should be Odyssey. If it's not there, you have to go to Options on the launcher, and log out. When you log back in again, be sure to use the steam option
5. VERY IMPORTANT: You must "link steam account" EVEN IF ITS ALREADY LINKED. This fixes what I think is a sync error between the launcher you have and frontier.
6. Repeat step 2, and check to see if Odyssey is now listed. It should be.