1 - remove battery
2 - connect usb cable to pc
3 - press and hold down u key on N900's keyboard
4 - reinsert battery (don't release u key)
5 - wait at least 4-5 secs (don't release u key)
6 - release u key only if usb logo is stable on screen

This should work and if not you have probably low battery (If battery charge is too low N900 doesn't activate flash mode)