Why isn't my MacBook running 64 bit mode?
I don't think these machines booted into 64bit. (the new macbook pros do).

You can hold down 6 and 4 whilst rebooting for a temporary 64bit boot (if available)

If that works, you can try this in terminal and then reboot

sudo defaults write /Library/Preferences/SystemConfiguration/com.apple.Boot 'Kernel Flags' 'arch=x86_64'


This will make the mac boot in 64bit mode by default.