mbs here u go
1.download the .deb version of the Flasher, install it with:
sudo dpkg -i <flasher-package-name>
If you are using 64-bit Linux, you'll need to force installation as follows:
sudo dpkg -i --force-architecture <flasher-package-name>
2. Type this: sudo flasher-3.5 -F <firmware-image> -f -R
3. It will say Suitable USB device not found, waiting•N900: While holding the u key, connect the N900 to your computer via USB. Wait until you see a dim screen on your N900 with a Nokia logo in the middle of your screen with an USB icon in the top right corner.
Hope this helps