Open stuffit and see the isupprt menu. Check for an update--I think there was an upgrade a while back that fixed something like the problem you describe.

You can also get that error if you tried to launch it before it was complete, or with some files ending in .img

If LW verified the complete file and your stuffit is up to date, there's not much you can do--a bad file was accurately downloaded. Please don't share that file.

Cheers.