Hulio, do you have same tuxguitar version in mac and windows??
i mean, suppose that you have
tuxguitar 1.0 in windows
tuxguitar 1.1 in macosx
when you save a file in mac version, it creates a "1.1" tg file format..
but when you try to open this 1.1 file in windows, the tuxguitar that you have installed don't know about the 1.1 existense.
If this is your case.. under mac version, try go to menu "File -> Export -> Export TuxGuitar 1.0"
If it's not the case.. do you have any way to give me a download link to the problematic file ?? so i can see what is happening.