This was posted on the EA forums, and seems to have fixed my problems:
To any Mac users encountering this problem, load and login, go in to the Community tab, click custom game, and launch the most recent item I had to do it twice, because the game froze when it was rebooting, and I ran into the "Losing Key Packets" error. If this happens, just do it again, and it should be fine.
My son's imac was having the "map loading partway and then freezing" problem, and I tried this and it fixed it.
I'm not sure exactly what it does, but thanks so much. I never even knew that was there.
Anyone else?