My understanding is that the new client hasn't yet been updated on Steam. The new client should support Mac. I suspect it will launch later today.
No hurry atm (a few annoying bugs/problems still) the old client still works so you can login, but its on its own old server so there are some players still there and some on the new and a few on both with dual clients.
@Sokolov I tried a few times to play through Steam, but this is the error code I get. T try the support, but nothing works.
I get the same message from Steam. Greens confirmed a couple hours ago that the Steam client has yet to be updated.