For all K64-capable Macs, boot while holding down “6″ and “4″ keys simultaneously to select the 64-bit kernel. For a more permanent solution, use the nvram command to add arch=x86_64 to your boot-args string, or edit the file /Library/Preferences/SystemConfiguration/com.apple.Boot.plist and addarch=x86_64 to the Kernel Flags string:
... <key>Kernel</key> <string>mach_kernel</string> <key>Kernel Flags</key> <string>arch=x86_64</string> ...
To switch back to the 32-bit kernel, hold down the “3″ and “2″ keys during boot, or use one of the techniques above, replacing “x86_64″ with “i386″.
