If you reboot, the 64-bit setting goes away. Here's how to enable it permanently, according to http://blogs.zdnet.com/Apple/?p=4716
Edit file /Library/Preferences/SystemConfiguration/com.apple.Boot.plist
Change this:
<key>Kernel Flags</key>
<string></string>
To this:
<key>Kernel Flags</key>
<string>arch=x86_64</string>
... and save the file.
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