1. account for gamecenter must be user/pass, not facebook
2. they must have gamecenter account verified and check it in Profile settings
3. They login to game with WOD client user/pass they've always used and click "enter"
4. New window pops open
5. LEFT SIDE, they put in their Gamecenter user/pass (cause no option for FB) and continue.
6. New window to continue as Gamecenter name, continue
7. They are in, make a post so they know they are inand it works (cause it will show logged in but not able to use unless they did all previous steps right). Log out
8. Open game client again, Login is email only, no password, click the MY button below empty password to login
9. Popup opens and they continue as Gamecenter account to game
10. From now on, they use this method to login
You really know its "game over" when the most fun we have is talking in forums...