When I try to open a file in Screenwriter 6 for Macintosh I only get the spinning wheel.

If the preferences file for the program gets damaged, it can cause issues in the program.  Renaming (or deleting) that file will cause the program to rebuild the file from scratch using the default settings.  This may correct the issue you are experiencing.

Note:  renaming or deleting the settings file will reset any global program options under Screenwriter > Preferences.  It will not change any document settings.

1. Quit out of Screenwriter under Screenwriter > Quit Screenwriter.
2. From the menus at the top, click on Go > Home.
   
   NOTE: If you are using Mac OS 10.7 (Lion), hold down the Option key when clicking on the Go menu and choose Library, skip to step 4.
   
   
3. Open the Library folder.
4. Open the Preferences folder.
5. Open the Movie Magic Screenwriter 6 folder.
6. Drag the Screenwriter Preferences file to the Trash.

Try relaunching Screenwriter to see if that solves the problem.