I have an iMac w/OS 10.5.6, a card reader and the SDM script for the Canon 570IS version 101A
Unfortunately, installing SDM or CHDK using a Mac is not as easy as with Windows.
Use a card 4GB or less in size and follow the instructions
here.
It is easier to use 'sdminst.exe' (supplied with Common_Files.zip on the SDM website) on a Windows computer to make a bootable card.
Let us know how you get on.
David