For dfu use you first need to install the Zadig driver.
http://zadig.akeo.ie/Connect your Revo with SBL pads soldered.
Download and Start Zadig software, you may see "STM32 Bootloader" displayed.
Click button "Install driver"
Download archive attached to this message
Decompress archive's contents
Double click the "Flash_revo.bat" file.
Wait until flash is finished.
Hit any key to close CMD window
Remove solder join between SBL pads
Connect to GCS