I'd run the command before attaching the USB cable, and rather than using their instructions would ensure that the device is not connected to anything (usb, power), power it down, then just plug in the usb cable (and it will automatically boot to flashing mode).

We are going OT here though, you may get more views (and therefore more people who've seen your problem) with a new thread specifically about the flashing question (with a title to match). I only use Linux to flash for example, so am just going from what I do there.