The activation code given by Amazon absolutely will activate the Mac Steam version. It did take me a little bit to figure out how to do it (I am a Steam Newbie): Don't waste time downloading the actual content! Instead:
In your Amazon Account, go to Your Account->Your Games and Software Library This will show all your Product Codes= activation codes In your steam log-in window, on the bottom LEFT, click '+ add a game' Enter your Product Codes= activation codes
Nothing to It!
(It took me about an hour to figure it out, so I am hoping I saved someone else some time...)
A few months ago, I got TTR for $1 from Humble Bumble with no DRM or Steam to deal with.
Play it all the time on Steam, I forget that Steam is even there most of the time. Humble bundle was Android only and IMO doesn't seem as polished as the Steam version and the AI also seems a little weak.