Look what I just found in another thread- it worked for me; hopefully for you, too! I just dragged the .sit file to the desktop, and it worked!
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chupa
Junior Member

Registered: Mar 2002
Location:
Posts: 1 Try this Installer fix for OS X...

Drag the installer app to your OS X desktop. Now launch it.

I was unsuccessful before I tried this because I was trying to install from another volume, not within my user directory.

-Chupa