Take a look into /etc/inittab, is there anything active with ttyS ?
The serial port must not be allways on, usually just when you send or receive data.
Is there anythin listening to the port, like a terminal, driver or so ?
Does your PC even know that there is something outside ?
Are the parameters correct: Same speed, parity, hardware-flow-control and so on ?
At last what have you added at the seriel port ?