I had the same problem. I just looked on the Steam website, searched for Football Manager 2012 and then found the demo and downloaded it. Only gives me the error when I click the link SI provide (or FM-Base, but it's the same link)
Turn your steam on and log in. then go to Start -> Run -> Type "steam://install/71280#" (without quotes). Your download should start in Steam. If you are using Vista/Win7 and you can't find run, press windows button and R, so windows button + R.
strange thing. cuz im residing in Canada, and if i go to the official website (NOW) to download, it redirects me to steam, and says 'unavailable'.
but whats strange is when it happen a few hours after release, which was late night. i just open steam is casualLY, and in the demo section, i see fm12, and i click it , and alas i GOT. i have fm 12 demo on my pc