Did you run `sudo xcode-select -switch /Applications/Xcode.app`? This message: `error: can't exec '/Developer/usr/bin/xcodebuild'` is telling you that your system is looking for xcodebuild in `/Developer/usr/bin/' but as of Xcode 4.3 (the one that's installed from the App Store) it's located in the Xcode.app bundle which is in /Applications.

**Edit**

In a comment to your question you say you've run `sudo xcode-select -switch /Applications/Xcode.app`. Now run `xcode-select -print-path` and make sure it says "/Applications/Xcode.app". 

Also, you may need to [download the command line tools separately](https://developer.apple.com/library/ios/#documentation/DeveloperTools/Conceptual/WhatsNewXcode/Articles/xcode_4_3.html#//apple_ref/doc/uid/1006-SW2). (I don't know from first-hand experience because I'm stuck on Snow Leopard for the moment so I haven't installed Xcode 4.3.)