Redownload the IPCC file. Once its on your desktop, alt click it (or whatever it is to bring up more info). Go down, delete .zip from the extenstion. Make sure its ATT_US.IPCC without the .zip.
That doesn't change it from being a zip file. Even after I remove the .zip extension, it's a zip file. If I then double click it, it opens a "Payload" folder that contains the ATT Bundle.
None of which is selectable to upload to the iPhone.
And the command prompt still does absolutely nothing when I type:
defaults write com.apple.iTunes carrier-testing -bool TRUE