How to get Arduino Mega 2560 Rev3 into DFU mode?

something wrong arduino mega 2560 r3. serial port option in ide greyed out of time , computer detecting when connect via usb. did research , found out might fault of 16u2 chip drivers. need use flip , go dfu mode fix problem. problem can't find clear instructions. 1 on arduino website r2 version. r3 bit different , i'm not sure how proceed. please

i have exact same problem. i'm trying use guide here when try putting mega 2560 dfu mode error:
dfu-programmer atmega16u2 erase
dfu-programmer: no device present.
make: *** [dfu] error 1

i assume means didn't arduino dfu mode

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