PRE projects are not interchangeable between platforms. Heck - they're not reliably interchangeable between versions on the same platform
Insanity is hereditary, you get it from your children
Welcome to the forum.
As Neale says, PrE's PREL (Project) files are not X-platform, though PrPro's (PRPROJ files) mostly are, with but a few exceptions. Now, this is but a theory on my part, and I'd love to have an Adobe engineer comment, but Adobe Encore's (full-featured DVD and BD authoring program) Project files (NCOR) are NOT X-platform, though Encore comes bundled with PrPro. The reason for that is that Encore is built on Sonic's Authorcore modules, that Adobe licenses, and those Authorcore modules differ pretty much for Mac, and for PC. Now, PrE, unlike PrPro, has some of those Sonic Authorcore modules, as it contains authoring capabilities, which was removed from PrPro at version CS3, the first X-platform version of PrPro. My guess is that it is those Sonic Authorcore modules, which kill PrE's X-platform nature.
Hey, if you had already created a complex composition using earlier version, I would suggest that you first export it, then transfer the exported file to your Mac and import it in PrE there.