I also have the MX500/IR543 combo. The trick to making the dim key (or any other) a repeating key is the way you learn the button to the mx500. If you hold the button down on the sending remote, the mx500 senses the repeating sequence and somehow knows to allow this key to be repeatedly sent when held down. If you are using the pre-programmed code in the mx500 for x10, you may need another learning remote (or one that has x10 codes that repeat) to do this programming.