I'm having this very same issue. I tried contacting Steam Support but I haven't received a response yet. I never opted into the beta so I thought maybe that was the problem. I purchased the PC version back in 2011 and recently noticed it became available for Mac so I'm trying to play it on my Macbook. I'm unable to install the game from my Steam library. Whenever I try to it presents the error message: "Terraria is not available on your current platform." I tried uninstalling and reinstalling Steam. Any ideas for what I can do?You have to download and install it on your Mac to play it. Installing Steam does not automatically install all the games you own, you have to do that separately.
As long as you are connected to the same Steam account where you bought Terraria for Windows, you should NOT have to purchase it again.
eta: now that I re-read what you posted: the open beta for Mac is closed now, it's fully released on Mac. If you took the steps at listed at the beginning of the thread, you should un-do them and get the full version. If you were trying to join the beta, then that is probably why you don't see it in your library.
Once we get your issue resolved, I will likely lock the thread.