Turn off your N900 pull the battery out, start all commands in maemo flasher until "suitable usb device not found, waiting" appears. Once it does plug in the usb to your N900 and computer (with the N900's battery out) then plug in your battery, wish for luck. Another way, is as you mentioned bus:address. You can know them by plugging anything say, flash drive into a certain usb port, then right click "computer" and click "manage." Go to Device manager located at the left of the Computer Management screen, and here is the tricky part. Expand Universal Serial Bus controllers and you will see a list of USB ports. Click each one to find the one that you inserted your flash drive to. After you find it double click it and look at "location:" it will tell you the bus and address. Once you know them, plug your N900 at the same port.
Example:
plugged in a sandisk cruzer into the back USB port of my pc so,
right click computer --> manage
at the right pane click device manager
on the window that appears click on Universal Serial Bus controllers which shows me a series of available USB ports
click each one in search of my sandisk
find that it lies in Intel(R) 82801G (ICH7 Family) USB Universal Host Controller - 27CB so double click this
then open General tab ( which is the one that opens at first) at location see PCI bus 0 device 29.
Then invite all my friends to party! Dunno why but okay...