cant get arduino mega 2560 to dfu mode

hi newbie here!

ive been trying to update my mega's 16U2's firmware using Atmel's Flip and am stuck in the bringing to dfu mode stage -
when i short ground and reset pins the arduino does disappear and then pops back as an arduino (not as an unknown device as should be) so i cant install Atmel's usb drivers.

the weird thing is that when i tried to do the same thing with my Uno R3 i wasnt facing any troubles.

i am using windows 7 x64 .

thanks!!
tali