Install utils for ext support on Mac OS X

1. OSXFuse – Download link
2. Fuse-ext2 – Download link

From post "Access Ext3/Ext2 file system" link

SD card setup on Mac for RP

From xterm run
df -h
to get
 /dev/disk1s1   3.8Gi  124Mi  3.7Gi     4%    /Volumes/CASIO-DSC

Check where the img file is...

 ls ~/Downloads/debian6-19-04-2012
 debian6-19-04-2012/     debian6-19-04-2012.zip

Unmount the SD card
 diskutil unmount /dev/disk1s1


use dd to copy image to SD card
 sudo dd bs=1m if=~/Downloads/debian6-19-04-2012/debian6-19-04-2012.ig of=/dev/rdisk1

link to relevant eLinux.org post