Mid-2012 Retina Pro - shouldn't I have Mountain Lion?
Depending on when you purchased it, it's possible that your machine came with OS X Lion preinstalled and you got the free upgrade to OS X Mountain Lion in July 2012. Apple's press release on the Mid-2012 MacBook Pro Retina dated June 11, 2012 states:


  The all new MacBook Pro ships with OS X Lion. Starting today, customers who purchase a Mac are eligible for a free copy of OS X Mountain Lion when it becomes available.


You can just visit the Mac App Store, sign in and install OS X Mountain Lion from there.