You need nVidia Inspector.
http://downloads.guru3d.com/NVIDIA-Inspector-1.94-download-2612.html
In nVidia control panel, go to Manage 3D Settings and press Add. Find rogame.exe which should be in X:\Program Files\Steam\SteamApps\common\red orchestra 2\Binaries\Win32
Set it to Override Anti-Aliasing setting to whatever you choose.
Now after you've installed nVidia Inspector, open it and click the tool icon to the right of the Driver Version column mid-way in the page.
When you have this open, up at the top you'll see a house icon. Click on it and select Red Orchestra 2 Heroes of Stalingrad.
At the very top under Compatibility column, third line down, set Antialiasing compatibility to Mass Effect, Mass Effect II.
Click apply changes, and now you're done.
Note: Weapon select screen will be blank in role menu.
Every page but Video will be blank in the options. For now these are unavoidable glitches when forcing AA via nVidia.