The best way to decompress files is to drag them directly over Stuffit Expander. I suggest you do the following:

1. Do a search for Stuffit Expander in Sherlock (Command F). I'd suggest you download the latest version (7) if you don't have it.

2. Command click its icon and put an alias on the desktop or in the Dock

3. Put a copy of your shared folder on the desktop as well (Applications>Limewire>Shared by default or do a Sherlock search)

Then whenever you want to unstuff a file go to the Finder, open your Shared folder and drag the icon over Stuffit Expander. If it's a valid, uncorrupted file, it should now decompress correctly.