Hello, other Susan,
Honestly, I had given up and installed the printer driver (the compressed file I had downloaded and couldn't expand) from an old disk instead. But then I received the Stuffit Expander suggestion and downloaded and installed THAT for future use. Since my printer driver problem was already solved, I hadn't actually unstuffed a file with it until now (figured I owed you some feedback on your question).
First, I went to my Applications Folder where the new Stuffit Expander was installed. I dragged the SE icon down into my dock (icon looks like a cardboard box with a white arrow in it). Then I dragged the compressed file out of my Downloads folder (that sits down by my Trash can) and into the "cardboard box" icon. Poof! The trouble was, I didn't know where the expanded file had parked itself. Fortunately, my husband was looking over my shoulder and advised me to look for it in that Downloads folder (he said it would appear next to the compressed file). There it was.
It appears to have expanded that compressed printer driver just fine.
My husband uses SE a lot, and he suggests you trash your Stuffit Expander download and go back to Alladin.com and download it again.
I agree, those numbered error messages are about the most useless thing ever.
I hope this helps and that it works for you. Unfortunately, I rarely need SE and so I am not an expert on it. But someone on this board will surely have a different solution if the new download fails to work for you. Good luck.