I just downloaded XCode 4.3.1. But when i build a project, i can not set its target to 5.1, as i can not find it. Also in the iPhone simulator, i can not find iPad retina there.
if you downloaded the xcode 4.3.1 from App store directly .. it directly install the xcode seperately try to search in your spotlight there must be 2 xcode now... if you start the new one it will ask you to remove the older version. remove the older sdk from there.
else check the current xcode version you are running. it is 4.3.1 or not.