The fact that you are getting that message means you are on the right track! I am assuming you are in Linux on a computer. Switch off the N900. Unplug the USB. Hold down the U key on the N900. Plug in the USB while still holding down the U key. Watch the n900 screen. It will be dim but you will see it. When you see the USB symbol, stop holding down the U key. Flashing should begin. More details in the Wiki.