I used Ultimate Boot CD and then slipstreamed WDIDLE3 into a new build of the ISO before burning it. So when booted, the program was in the DOS utils folder on one of the mounted virtual drives.

You can follow the instruction here: http://www.xlr8yourmac.com/tips/Disable_WDGreen_HeadParking.htm

It's not the site I used as a guide, but the instructions are pretty much the same. Keep in mind that the version number referenced in some of the commands will be different since the current version of the boot disk is newer (5.xxx or something)