I was stunned the other day when I was looking through my system info on my Snow Leopard equipped Mac Book Pro to find it was running in 32bit mode…. Turns out you have to manually configure it to run in 64bit every single time you boot up (as seen further below), or by holding down 6 and 4 on the keyboard at startup.

The permanent fix:

Edit the file:

/Library/Preferences/SystemConfiguration/com.apple.Boot.plist

Insert arch=x86_64 into the Kernel Flags field.

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>Kernel</key>
<string>mach_kernel</string>
<key>Kernel Flags</key>
<string>arch=x86_64</string>
</dict>
</plist>

via Macintosh Performance Guide: Mac OS X 10.6 Snow Leopard: Booting the 64-bit Kernel.