I think that is the error I often get when I try to open a Stuffit 5 file on a computer that can only handle Stuffit 3 or 4. I haven't taken the time to totally chase down what the requirements of Stuffit 5 are, I usually just move those files over to my OS9 machine and unstuff them there. Try installing Stuffit 5. If your computer says it can't install Stuffit 5, see if you can move the files to a computer with a higher level OS.