Repairing permissions isn't the answer to everything, you know 🙄 .
To the OT: you need to download Stuffit Expander. Then, you right click (or option click) the file you want to open, and select Open With. Then you choose Stuffit Expander to open it. If you want you Mac to remember that setting next time, check a box that appears in the bottom of the window when you're selecting the app.