Hello!
Which version of Stuffit are you using?.
I would recommend not to use the pay version instead go and download and install the free Stuffit Lite 6.5.1 package.
http://www.stuffit.com
Then use the free Stuffit Expander to unstuff the .sit files.
If you are trying to open them from within the LW Shared folder then that might be your problem instead move the downloaded file to the desktop and realese it ontop of the Stuffit Expander alias icon and it should unstuff just fine atleast if the file have the proper .sit file extension added to it.If not then first open up Stuffit and from whitin the program go to the File menu and select the option Expand and then browse to your file which should be on the desktop and click choose/expand and it should expand just fine.
If not then the files you have downloaded might indeed be corrupted or it maybe something wrong with your computer.Check the integrity of your HD with Disk First Aid and also rebuild your desktop and zap PRAM using Tech Tool Lite.