It seems that Apple’s newer Archive Utility, the default program instead of Stuffit, seems to have problems with some of our older installers.
Right-click (not double-click) on a downloaded file, select “Open With”, then select Stuffit Expander. You should see a blue ViseX icon with the caption of "Collection xxxx" where xxxx is the number of the collection. Double-click the ViseX icon and the installer should open.
If you don't have StuffIt Expander you can download it for free at http://my.smithmicro.com/mac/stuffit/expander.html. This link works correctly as of May 2009.