Premiere Elements' project (.prel) files are not cross-platform.
You can not open a project that was started on a Windows computer with the Mac version of the program.
Thank you for your reply. However, I have since loaded a few other other videos I made on my Sony laptop and two of the videos do run.
The project may have effects not available on the new version and thus not be compatible.
Bob also makes a good point! The Mac version does not include all of the effects the Windows version has.
As I've said, these project files are not cross-platform.