Uugh, I hate when that happens. Someone sends you a corrupted package -- corrupted because it either wasn't packaged properly, or only part of the package was downloaded. So in an attempt to expand the file (which normally works with Stuffit), you can't.
In some cases, though, packages are made from other platforms and file types that even Stuffit can't handle them. For instance, in late 2003, I downloaded a couple of 3 to 5MB .rar files, but Stuffit Deluxe (v8.0.x) couldn't open them. I had to use a program called UnRarX to get at those nasty suckers.
So watch out for things like that.
In the case that you're pretty sure a file isn't corrupted, then go to Versiontracker, do a search that might return programs that would extract the file for your file type (I found UnRarX by doing a search with the query "rar"), and do some trial and error with the resulting programs.