So, you might notice that Steam for the Mac doesn't yet include Tidalis. Never fear, Tidalis is coming to Steam for the Mac, but I'm not certain exactly when. We and Valve had hoped it would be today, but we ran into a technical issue that we haven't yet solved getting Tidalis to run properly from Steam on the Mac. Myself and one of their engineers (intrepid soul to whom I'm intensely grateful!) stayed up literally half the night last night trying to get this resolved, and while we did get very close, there's still a remaining issue we're trying to sort out.
At any rate, as soon as this issue is solved there, we'll have Tidalis out on Steam for the Mac. Note that the other Mac versions available elsewhere are not affected by this at all, this is just limited to our integration with Steam at the moment.
We'll update you as soon as it's ready!