Please keep in mind, you must have a basic understanding of scripts and how they are used to be able to complete this project.
If you've already set up Airport Express and are able to stream music to your stereo using iTunes, you may find that it can be a hassle going over to the computer every time you want the next song to come on. This is especially time consuming if the computer is far away from the listening area.
Set up the KeypadLinc's small buttons to control "Stop," "Play," "Previous," and "Next" (leaving the large buttons to control a light). When the KeypadLinc sends your command to the PowerLinc Controller, the PowerLinc tells the Indigo automation software to execute Apple Script, which in turn tells iTunes to execute your command.
If you have Mac OS 10.4 Tiger, you can use its simple "Automator" program instead of Apple Script.