Is filemerge still available after Xcode 4.3 installation?
In Xcode, choose this menu item: Xcode -> Open Developer Tool -> FileMerge. The app itself is here:
/Applications/Xcode.app/Contents/Applications/FileMerge.app
On the command line, use opendiff
. You must have the command line tools installed -- go to Xcode, Preferences, Downloads tab, and install Command Line Tools if you haven't already.