which os is ur pc? this didnt work for me in winxp. try Win7 also pls dont use the " inverted comas along with the commands.

Press and hold U button in the n900 key board and connect it with the pc .Keep holding the button until u see NOKIA logo with out the back light and the usb icon on the top right corner. then release the U button and run the command in maemo flasher.

Good luck.