Datasheet doesn't include pinout info?
It sounds to me like you're going the wrong direction with the hardware you have available.  If you're using a USB network adapter, it's probably one of those where you have to install drivers in the OS to make it work.  And if you have to do this, then you've got your work cut out for you trying to interface an Arduino with such a USB device.  It's not going to be as simple as an HID device (i.e. keyboard or mouse).

If you have an Arduino, why not stick with one of the tried-and-true solutions, like an Ethernet Shield?  Or try the SPI device that Jeremy Kerr has mentioned in his answer.