do you have steam open and are you logged in?
I always have Steam open and I'm always logged in.
Consider that Tuesdays are Steam maintenance days, so if you catch the servers in downtime you won't be able to authenticate.
Did you try restarting steam?
Look forward to jumping back into this game when you support 64-Bit.
When's 64-bit coming? Mac no longer supports 32-Bit and I doubt windows will support 32-Bit for much longer...I want my money back otherwise ;) JK.Look forward to jumping back into this game when you support 64-Bit.
Speaking of macs – the game is unable to authenticate on OS X, giving the error "Authentication failed: Build too old".