There's a "forced" way to enable AA with Nvidia cards but I don't think there's a path for ATI yet.
Just if you want to try with NVIDIA, what you need to do is:
For reference, gotta disable AA in the game settings, exit game, go to Nvidia Control Panel and under 'Manage 3D Settings' select 'Clive Barker's Jericho'.
Set up something like:
Antialiasing - Mode: Override
Antialiasing - Setting: 2x
Antialiasing - Transparency: 4x (supersample)
Copy the file Mafia2.exe found in \Program Files (x86)\Steam\steamapps\common\mafia ii - public demo\pc to a Jericho.exe (or just rename), make sure Steam is running, and start Jericho.exe.
I've looked for any other responses in many forums but didn't find any solution for ATI GPUs at the moment.
Perhaps, you could contact ATI and hear if they have a hotfix or something.