As the person who posted the original Wishlist message that started this, first can I say thank you to raybeau528 for his solution.
Having belatedly followed the link through from my original thread, I too encountered the same error message as uksnowy however ...
The error message only appears if you use Tools->Scripts.
Instead of this, set up the parameters for PlayListBuilder using Tools->Options, thereafter all the (useful) functions of the script can be accessed by right clicking on different Playlists, first right click on each of the Playlists you want to add to the mix, and from the PlayListBuilder right click option choose Add Selection. Finally right click on the parent Playlist, and from the PlayListBuilder option, choose Build Sub-Playlists.
So PlayListBuilder is fully useable despite the error message.
Note To raybeau528: Thanks for this, PlayListBuilder is just what I need, but if there is another version, could you please fix the Tools->Scripts problem. (FYI I'm running Windows XP and MediaMonkey 220.127.116.110)