Go to Steam-Help-System Info, and paste everything there into here, except the Installed Software list at the end.
Please attach your Documents/Paradox/CK2/settings.txt. From your Documents/Paradox/CK2/logs/ directory, post system.log (rename to system.txt to attach). What version # and checksum do you see a/ in the launcher and b/ in the main game menu, when loaded with NO mods active?
File Locations on Mac OS X
The executable is in ~/Library/Application Support/Steam/SteamApps/common/Crusader Kings II/ck2.app
The user files (settings.txt, error logs, saved games, gfx) are in Users/<UserName>/Documents/Paradox Interactive/Crusader Kings II/
To see ~/Library/ go to the Finder, hit command-shift-G (ie Go To) and type in ~/Library/ .