Multicolor Plugins are gone on Mac OS Photoshop CS2 or CS3
In order to operate the Multicolor plugins on CS2 or CS3, users will need to first open the application under PowerPC Emulation mode, (also referred to as Rosetta.) If using an Intel-based Macintosh, in order to use the plug-in first run Photoshop under Rosetta:
Navigate to Applications/Adobe Photoshop CS3.
Select the Adobe Photoshop CS3 icon and choose File > Get Info.
In the General section, enable the Open Using Rosetta option.
Rosetta is a dynamic translator for Mac OS X that is distributed by Apple. It enables applications compiled for the PowerPC family of processors to run on Apple systems that use Intel processors. It is a key part of Apple's strategy for the transition of their Macintosh line from PowerPC to Intel processors as it enables pre-existing Mac OS X software to run on the new platform without modification.
Here is a link to some additional information about Rosetta Mode: