The xcode project files in this directory are contributed by Andy Maloney and are not continuously updated.
Currently, it's for Assimp r352. If you're using a newer revision, watch out for missing files/includes/...

See Andy's description at
http://sourceforge.net/tracker/index.php?func=detail&aid=2659135&group_id=226462&atid=1067634 (Tracker item 2659135)
for more information.