You don't turn it on when flashing using 'Flasher 3.5' from a PC. Yank the battery, put the battery back in DONOT turn on the N900. Connect the USB lead to the PC and open the keyboard on the N900. Hold the 'U' key down on the N900 and at the same time plug in the other end of the USB lead into it. Keep your finger on the 'U' until the N900 screen displays dimmed the blue 'NOKIA' in the middle and the symbol for USB in the top right. It's dimmed so can be hard to see.

When the N900 displays this it is in 'flashing mode' it will stay like this forever, unless you yank the battery again. Note: it doesn't charge in this mode. Now follow the instructions given about what to do with 'Flasher 3.5' and it should be fine.