Boot Once Into 64-bit Mode

• 1). Power on the Mac.
  
  
• 2). Press and hold the "6" and "4" keys simultaneously immediately.
  
  
• 3). Release the keys after the screen turns blue.
  
  

Always Boot Into 64-bit Mode

• 1). Open a Terminal window.
  
  
• 2). Type "sudo systemsetup -setkernelbootarchitecture x86_64" without quotes, then press "Return."
  
  
• 3). Enter the administrator password, then press "Return."
  
  
• 4). Restart the computer.