Stuffit Will Not "Expand" .sit Files

Please help. Every time I double-click a ".sit" file (or, select Stuffit from the "open with" menu item), a small windows opens for less than a second, then closes. Nothing else happens. I can't tell what the window is because it only flashes on the desktop.

I am experiencing no other problems on my PM. I have deleted all the Stuffit files I could find using Spotlight; Repaired Disk Permissions and Repaired Disk (no problems found in either); re-booted, and re-downloaded and re-installed Stuffit Expander -- but, I get the exact same results.

It must be something simple that I am overlooking, as I am fairly new to the Mac world. Any help would be greatly appreciated.