Hello. I bought a Arduino Uno R3. After installing the Arduino software / IDE, I coulnt coonect it to the Arduino Board. In device manager I have a yellow triangle with ATmega16u2 DFU and we cant update the Drivers. I do not know, what is the problem. We have tried it on Windows 7 / Windows 8.1 both professional.
Not quite my area of expertise but it sounds like the ttl-to-usb chip was not programmed with its firmware.
There are instructions on the web how to burn the firmware (search e.g. for atmel flip); I've never done it so don't know what's involved.
Alternatively send it back to the supplier and ask for replacement.