late reply but this is the answer:
"You need to boot your device into the bootloader interface. To do this, you need to hold the volume down button while the device boots.
The easiest approach is to reboot the device and begin holding the volume down button until it boots up into the bootloader interface.
Alternatively, turn off the device, then boot it up while holding the volume down button during the boot process. You can either boot it with the power button or by plugging it in as required in the next section.
This step is not complete until your device displays a red warning triangle and the words "Fastboot Mode". You must not press the device's power button to activate the "Start" menu item, because the device must remain paused in Fastboot mode for the installer to connect to it."