The latest Stuffit Expander (the free download not Deluxe) should open all. The .zip files may be tricky. Make sure it is a Mac file and not a windows file.
You may need to select the file and tell your computer what program to use to open it. You probably already have Stuffit Expander on your hard drive, so when you Get Info on the file, make sure you select Stuffit Expander as the program with which to open it.