8u2 programming with Flip 3.4.5
When you attach the device to the USB port - does it install DFU drivers? If it does not there are 2 things that are possible: 1) you're not booting the device in DFU mode, 2) drivers are not installed. I saw your other question here Reset 8u2 on Arduino Mega 2560 so if you haven't resolved that you didn't go far enough to have problem 2).

So - it needs to be in DFU mode first :)