It looks like the ATmega16u2 is in DFU mode. Meaning the ATmega16u2 does not have the serial firmware loaded.
This should have been done in the factory.
Anyway, you can attempt to load the serial firmware yourself or you could try to exchange it at Radio Shack.
If you attempt to upload the firmware yourself, you will need to get a patched version of dfu-programmer that supports the ATmega16u2.
The current version of dfu-programmer on Sourceforge has not been updated to support the 16U2.
Here is a link to a forum post about patching and building dfu-programmer:
Once you have a working version of dfu-programmer, you can find the firmware in your Arduino download:
hardware\arduino\firmwares\arduino-usbserial\Arduino-usbserial-atmega16u2-Uno-Rev3.hex