1. Launch TextEdit 2. Open the file named /Library/Preferences/SystemConfiguration/com.apple.Boot.plist 3. add arch=x86_64 to the Kernel Flags string: … <key>Kernel</key> <string>mach_kernel</string> <key>Kernel Flags</key> <string>arch=x86_64</string> ... —- Then to switch back to the 32-bit kernel, hold down the “3” and “2” keys during boot, or replace “x86_64” with “i386” in the .plist file. (source)