Just a really quick hit, because it took me a little while to find this post, which pointed me in the right direction.
If you’re trying to use the PowerLinc 2412U on OSX 10.5 with the latest beta (4.1) of the Indigo home-automation and control server, you’ll need to install USB serial drivers first. It would have been nice if this was a little more apparent with either the Indigo software or the PLM’s documentation, but it’s not (or, if it is, it’s buried).
Get the USB serial drivers from here, install them, and now you should be able to see the interface and the proper port in the preferences panel. Hopefully this will save you a bit of the aggravation I went through trying to figure out why the hell I couldn’t initialize the interface between Indigo and the PLM.