Supported Platforms of the Native Libraries Guide documentation in Apache Hadoop reads:
The native hadoop library is supported on *nix platforms only. The library does not to work with Cygwin or the Mac OS X platform.
The native hadoop library is mainly used on the GNU/Linus platform and has been tested on these distributions:
- RHEL4/Fedora
- Ubuntu
- Gentoo
On all the above distributions a 32/64 bit native hadoop library will work with a respective 32/64 bit jvm.
It appears that the WARN message should be disregarded on Mac OS X as the native library doesn't simply exist for the platform.