https://xenonfossil.itch.io/creature-creation-station/devlog/1454665/steam-keys-...

Here's the devlog with the instructions! You'll have to type in the itch link provided in the tiny envelope in your physical case first, to attach the game to your itch account, in order to be eligible for claiming the Steam Key. Let me know if you're still having trouble though! I think a few of the early itch codes might be glitching when you try to get an external key. 

I redeemed the Itch key about 2 years ago. Unfortunately, the Steam Key option doesn't seem to show up under My Library>My Purchases>Creature Creation Station.

I tried having Itch.io resend the download links, but it only sent links for games I paid direct money to the site for. Anything I redeemed for free or via a code (like in Creation Creation Station's physical box) didn't appear in the email. Which is weird, because I can download it from my library.

Maybe Itch only forwarded the Steam Keys for the game when  bought digitally?