Potential causes of communications trouble are:

• the controller is not connected to the Macintosh,
• the cable between the controller and Macintosh is not the correct one,
• the controller cable is not connected to the same serial port as Thinking Home has selected,
• other software is trying to use the same serial port, making it unavailable to Thinking Home.
• the controller cable was temporarily disconnected. See details for more information.

Also, check out the other info on troubleshooting communications.