You Need an RS232 Connecting Cable & Adapters As Necessary
Universal offers a pre-made cable suitable for connecting a Serial port to a Home Theater Master remote control.
However, should you wish to make your own, here is the pin configuration for a serial cable:
DB 9 ,DB25 Function Stereo 3.5mm Plug Connection
Pin 3 ,Pin 2 Host Transmit To Tip
Pin 2 ,Pin 3 Host Receive To Middle Ring
Pin 5 ,Pin 7 Host Ground To Base Ring

Straight from the URC website download
[Link: urccontrolroom.com]