Did you follow this basic step?
1. Power off the device
2. Connect Usb cable to computer.
3. While holding the U key on N900 querry keyboard, connect your N900 to the cable and wait until the Nokia sceen come up with a little usb icon on top right of the screen and release the key.
4. Now using flasher to flash your emmc bin file first and make sure don't use the -R which reboot the phone.
5. after done, remove n900 from the cable and remove the battery and put it back in then follow just step 3 above.
6. Now use flasher to flash N900 OS bin with -R to reboot the phone after it's done.