Chances are, your question has been asked before and the answer is here. If not, you can also check the Thinking Home community forum.
Q: My controller is not found or not responding.
A:
By far, the most common problem reported is that the home automation serial port is not found on Mac OS X. This is due to undesired default behavior of Network Preferences and is easily solved.
Even though you’re relying on Ethernet or Airport for networking, new serial ports are checked for modems and this confuses most X-10 controllers.
You want to make your serial port unavailable for networking so it will become available for home automation. Go to System Preferences, select Network. The next steps depend on which version of Mac OS X you are using.
This solution is for Leopard (10.5). Details for Tiger (10.4) and Panther (10.3) are similar.
Select your serial port from the list on the left and delete it by pressing the (—) button below the list.
You don’t want the OS networking even peeking at the port, asking modem-like questions of a home automation controller that can get confused by the unexpected data.
If this makes the port found and the controller is not responding, it is probably confused and needs all power removed for 15 minutes.
These steps will get you back in operation again.
This is the answer.