I just got a second mega R3 (but with CH340 USB chip). When I plugged into a usb port Windows 10 reports 'USB device not recognised'.
I can see this seems to be a common problem but cannot find a Windows 10 solution that works. There was one solution that implied I should have a Mega driver in Arduino/drivers, but I only see FTDI drivers there and some setup files e.g. Arduino.
I (think) I have already loaded CH340 drivers (about 6 times), but because the Mega is not recognised, I do not think my pc gets far enough to load the CH340 drivers.
I am at IDE 1.6.5