title: How can I add older version of iOS SDK in Xcode 4.5
body: <p>I copied the iOS 5.1 SDK here: </p>
  
  <blockquote>
    <p>Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs</p>
  </blockquote>
  
  <p>But, I still can't choose the base SDK in Xcode.
  Can anybody help?</p>