what does the phone manual say ?
or has this worked in the past ?
Have you tried turning the phone off, plugging in the usb cable to phone and computer, then turn phone on ?
or you may have to change the mode the phone is in to get it to act like a disk drive.
have you tried different usb ports or cables ?
i had one where the the usb port was so worn that i had to hold the cable up on the phone end to make it connect.