DFU on UNO R3 issue

This is really anoying me, I bought a Pump It Up PAD and I´m trying to port it to a PC VIA UnoJoy, I followed step by step this tutorial: https://www.instructables.com/id/Control-Arcade-Arduino-UNO/
and when I get the part of DFU my arduino UNO, I join the pins but the “L” LED only flash a couple times, then turns off, my pc on “Device Manager” recognices ATmega16U2 on “Atmel USB Devices”.
I really need know why I can´t turn on the DFU mode on my arduino.

thanks a lot to read it <3
(srry my bad english)

It sounds like you corrupted a bootloader.

Burning a new one has a few methods. Another Arduino

Or something like an AVRISP.

Bob.