Mega 2560 Bootloader Mode

I have been trying to update the firmware on my mega2560 to MOCO for LUFA using Atmel Flip.
I placed the Arduino into DFU programming mode and followed the instructions for Flip.
However, the USB port will not open. I contacted Atmel support and they said to place the arduino into Bootloader mode.

Does anyone know how to place a mega 2560 into bootloader mode?

Any help would be greatly appreciated
thanks.