StuffIt Expander does work for macs. It started out as a mac only program but now there is a windows version. You haven't told us your operating system version as required by the forum rules so I don't know which expander to direct you to. Try here:
http://www.apple.com/downloads/macos...texpander.html OR here:
http://www.stuffit.com/mac/expander/index.html
StuffIt Expander version 9 can only be used for OS 10.3 or higher. For Mac OS X 10.1 - 10.2 - Version 8. And for Mac OS Classic 8.6 - 9.x - Version 7.
Your Mac OS install disk will most likely have a copy of StuffIt expander on it. So just re-install the appropriate version from there & go to the above site's update page to get an appropriate update if needed.