title: How to add base SDK for 10.5 in xCode 4
body: <p>I've installed xCode 4 last night and tried to compile a software which should support 10.5 and above OSs.</p>
  
  <p>It gave the error message "Base SDK missing". I found that 10.5 base SDK is no longer in the menu.</p>
  
  <p>Is there any way to get that base SDK? Is there any get around for this problem?</p>
  
  <p>Thanks in advance.</p>