I know nothing about OSX, but there must be some sort of pkgconfig file that is SUPPOSED to be setting the version number. I think the easiest way to find out what version the system is reporting is to add the following command between if (ASSIMP_FOUND) and if (NOT ${ASSIMP_VERSION} VERSION_LESS "2.0.1150"): message ("ASSIMP_VERSION:${ASSIMP_VERSION}")