USB to serial converter not available to Win7 Guest

I am trying to use a serial to USB converter (Aten UC232A) with a Win 7 Guest on a Win 7 host, with Workstation 9.

The converter does not show up as a removable device that I can connect to the guest.

I also tried to let the host OS set it up as a serial port (COM5), and then add a serial port to the guest OS.  In the guest, I am not given COM5 as an option to map the guest serial port to.

Any suggestions on how to resolve this?

Thanks!