Hi there, no problem - it sounds like you're using the itch.io launcher, so you might need to sign into the web browser instead. On the web browser if you log in and select your account at the top right, then go to "My Library", "Things You Own" and then hit the red download button on the game. It should take you to a page with three builds available and beneath that is a button that says "Get Steam Key".
Again, I don't think you'll actually be able to play it on Steam until it launches there on Oct 8th but it sounds like other people have been able to activate it in advance!