This may be because the N900 does not support USB host natively, but if you do a do a quick search then you should find the USB Host BETA thread which may resolve this for you?

Other options for you are either using the Headphone jack (+Cable if you have a port) or the absolutely AWESOME FM transmitter App that is available via the app manager. This can not only transmit your personal tunes but also tie in with internet Radio and transmit 100s of radio stations in to your car or any other RF device.

A much better option IMO