We traced the checksum difference that we're getting between the MAC and Windows to a special character that's at the beginning of the unit.txt and technology.txt files. If you replace those two files in your install with
these ones then the false checksum failed error should go away AFTER one of you has played a turn and both of you have updated to the two new files. This means that after you both update whomever plays the next turn will still get a false checksum failed messages on these two files but after that neither of you should get it again if you both continue to use these files.
