Did you start the flasher app on your computer first, before you powered up the N900 and pressed the "U" key to get into flashing mode?

Try starting the flasher app first (it will tell you device not found, waiting for device - or some such thing), THEN power your N900 + U key, THEN plug it into your computer's USB port. If all goes well, flashed should pick up the N900 and start flashing it.