Where is the Apple Air Port Utility 'Use dynamic global hostname' option?
Apple seem to have removed/hidden some of the more advanced options from the Airport Utility interface in version 6 and this is likely one of them. Unless there's a workaround someone else can suggest to make such options visible again, on 10.7 (Lion), I've been getting around this by using the AirPort Utility 5.6 for Mac OS X Lion. You can find the Dynamic Global Hostname option by clicking Manual Setup > AirPort > Base Station tab > Edit - though I'm sure you already know that :)

Note that I have the AirPort Express 802.11n (2nd Generation) and the Airport Utility 5.6 does complain that


  This version of AirPort Utility doesn't support this AirPort wireless
  device and might improperly configure the device if you continue to
  use it


when you try to configure it. It seems to read all the information correctly but I would urge you to export your configuration first and/or not have anything you can't put back after a full reset, if things go wrong.