Any SDHC reader will work as long as you have the patch for older OS’s installed. The issue is not hardware incompatibility is is support for exFAT file system that is the compatibility issue.
On MAC technically 10.6.4 is correct but only builds made after July 10’. Apple did not add exFAT support to 10.6.4 builds until after July. Best bet is up update to the latest MAC OS which is 10.6.6 and you will have no problems reading your SDXC card.
http://blog.fosketts.net/2010/10/01/sdxc-exfat-apple-mac-os-imac-mini/
