I know of no freeware genealogy programs for the Mac-sorry.
Can your Mac run a Windows simulator?
If so you could get one and then grab a copy of PAF of Legacy, get a GEDOM from you cousin and a GEDCOM of your online tree- import them both into PAF, eliminate duplicates, do clean up, etc and then export that file to a new GEDCOM.
At least you'd have everything in one GEDCOM. Then you can try to find a way to get that GEDCOM onto your Mac.