If you don't have the previous version 6.5 archived somewhere yourself then your best bet is to try searching the web. All you need is the free stuffit expander 6.5 so maybe it is possible there is a site out there that has it available for download. Try a google search
www.google.comSorry I can't be more specific, but it seems that all companies, that come out with new software now delete any existence of previous versions from their websites so that users that upgrade have to be stuck with problems and are forced to post them here. It's a win win situation for software companies, because they get immediate feedback on their new product because people are forced to deal with it.
BTW, I have found that if you are like me and expected Stuffit Expander 7 to be working correctly and kept trying to use it to expand an archive because I thought something was wrong on my end you may have a bigger problem then it just not working. When I was trying to unstuff the same file a few times and I didn't see any results, it was because the results were invisible files which I think someone mentioned in an earlier post. The bad part is, if you are using a Classic Mac version their is nothing built into the finder to show invisible files. I recommend finding an application that will show invisibles and maybe even something like "FileBuddy" which allows you to delete invisible files. To my surprise when I looked in my downloads folder where I was trying to unstuff files with Expander 7 I had a lot of invisible files and even some invisible aliases of files. I don't know about you, but this was annoying since I couldn't have things with the same name in the folder because there were invisible items with that name already. Just another bit of info.