Hi brpubs
Some of the older versions were compatible across the different platforms but I'm not sure if this would work for the really old files. If you are still using Mac and have upgraded your IOS, then you may not be able to use MYOB on it (except for online versions). I'm not familiar with Mac generally, so can't advise much on this. If you are really needing a solution quickly, then maybe it's worth contacting FGH Tables to see if they can assist - third party but their fees are reasonable and my clients and I have used their services for a number of years without any issues.
Cheers
Liz