make sure the battery is fully loaded,
unplug all cables,
turn off,
remove battery,
connect USB cable from N900 to PC,
start flasher on PC,
slide out keyboard,
hit U, and keep it pressed while you plug the battery back in
wait until you see the NOLO screen ("NOKIA") and a small USB symbol on top right,
flasher on PC should recognize the N900 and do the flashing