I think National provides an example package on their website. Or maybe you can ask them for it. Basically, your device needs to do 2 things.
1. Interface the chip's registers. This stage basically queries chip registers on receiving interrupts. All these information is presented in the datasheet.
2. Handle the usb protocol. You'll need to know the usb protocol for this. You can download the usb specifications from www.usb.org.