The 9-pin D-Sub connector on the 430 MkII ADC card is not a standard 'serial' port and does not send data that a serial port on a computer would receive and understand. It can be switched between sending AES and S/PDIF digital audio signals. You will most likely need a breakout cable to use it. Focusrite manufacture two different breakout cables - one with XLRs for AES and one with RCA phono connectors for S/PDIF. You can purchase these breakout cables from any Focusrite dealer or distributor.
The pin-out for the cables can be found in this document - ad9pinconfig.pdf