If your system has a mishmash of MySQL installs like mine did earlier today, it makes sense to start with a clean slate. I had the default that comes with xcode, the version that comes with MAMP, and then one I had tried to install from source. After running Oracle’s GUI installer, none of them would run. At this point, a fresh start made sense – I wiped all traces of MySQL from my system and started from scratch with the GUI installer. Here are the details from Rob Allen.
If you’re running MAMP, it might help to nuke that too.
Additionally, I recommend getting rid of old config files:
rm /etc/my.cnf
rm /etc/mysql/my.cnf
rm /usr/local/mysql/etc/my.cnf
rm ~/.my.cnf