You have to delete the folder named “Cubase 6” under “/Users/(your account)/Library/Preferences”.
In case you have an older Cubase version installed you would also have to temporarily rename any older Cubase preferences folder that can be found in the same place (for example a “Cubase 5” preferences folder) in order to prevent Cubase 6 from importing the old settings again upon the next launch.
By the way, on my system crossfades work absolutely fine, but I have one question:
You write that “events will only overlap when pushed into each other”. I assume that you tried to put two audio events one after the other, then selected them and then pressed “x” in order to make them overlap and create a crossfade. Are you sure the audio events in case can be expanded to the respective sides, that means the left event end boundary to the right and the right event start to the left? If both events don’t contain any more information on the respective sides then the can’t be expanded anymore and no crossfade area can be created. I’m sure you actually know this but maybe you forgot to check the audio files…