Mac OS X is an operating system created by Apple. The Graal client is currently provided in a PPC binary for use on pre-Intel Macs. Most Apple computers today use Intel processors, and therefore Rosetta emulation is required to run Graal v5, severely hurting performance. Graal v5 cannot be run on OS X Lion since Apple has dropped Rosetta support.
The v6 beta (but, officially, "stable") Graal version is distributed in a universal binary which supports 32-bit and 64-bit Intel processors, as well as older PPC computers.
See also
Windows
Linux
iPhone
Gameboy Colour