SOLUTION for those unable to select DX10/11 or with greyed out AA
All - i'm running windows 7, 64 bit. Couldn't wait to get home, launched Civ5 only to find I can only run DX 9, and on top of that... unable to select any AA options.
Found a solution after fiddling around. Do the following:
1) Go to the location of the civ 5 install (mine is under C:\Program Files (x86)\Steam\steamapps\common\sid meier's civilization v)
2) Look for the file Launcher.exe
3) Right click Launcher.exe, and select properties
4) Go to the compatibility tab
5) Check the option "Run this program in compatibility mode for:", and select Windows Vista Service Pack 2 (try other options if this is your current OS, I suppose)
6) Click to accept/save
You can either launch the game from launcher.exe, or from steam or any shortcut. Since we set compatibility on the main exe, all calls to it shall invoke the compatibility goodness. This won't "degrade" your performance at all - it simply tricks the OS into sending the correct OS codes. Some apps like to be fiddly with them. Giggity.
This also enabled AA for me - probably a byproduct of being above DX9.
Anyways, if this helps - drop a thanks! If not, stop by anyways, i'm up all night.
Just a small bump, saw a few other posts with users having this problem.