Problem with DFU mode in Arduino Uno R3

Hi, First of all, I would like to thanks for any help you can give me. I recently bought and Arduino Uno R3 microcontroller for a project, now I have to flash it to use it as a keyboard but my biggest problem is when I want to set the DFU mode

I'm using this tutorial , my problem is in step 3 from http://projectsfromtech.blogspot.co.uk/2013/03/updating-firmware-on-atmega16u2.html

says "The Arduino should disappear from the device manager. Take the wire off. The Arduino should re-enter as an unknown device. It is now in DFU mode."

when I take the wire off, doesn't re-enter as an unknown device, re-enter under Ports (COM & LPT) not in DFU mode and I have tested in 3 pc and all the same.

I'm using windows 7 and I have installed all the latest version of drivers and FLIP