Yes, you will need an IR543, it takes the X10 IR signals from your MX-500 and puts them on the powerline so your X10 devices, switches & modules, can do their 'magic'.
A transceiver takes the RF signals put out by X10 wireless switches and X10 brand remote controls and sends them via the powerline to your X10 devices.
Check out these links for more information on X10:
[Link: homeautomationnet.com][Link: pigselectronics.com]http://www.x10ideas.com/