Dave's recommendation is exactly right. If FTM needs a Gedcom 5.5 file, you can import it into software that can read the older gedcom format and then export the data in gedcom 5.5. format.
In fact PAF5 can do exactly that. If you use extensive notes in your data file, I remember hearing that going through PAF4 then PAF5 gives better formatting of the notes. If you do not have lengthy notes, just try PAF5.
If all else fails, email a copy of your Mac PAF 2.3.1 file to email@example.com with "Disk Doctor" as the title, and provide instructions on what you want done. The service is free and comes without any guarantees.