I can't add tracks to any of the playlists that I've created!

Are you creating playlists with Musicmatch or are you creating them directly on the DJ?

Let's say you have a playlist on your DJ and want to add a track to it. First, play the playlist. While it is playing, go to Music Library, find the track you want to add, select it with the thumbwheel, one of your choices in the menu that comes up is "Add to Selected". Select "Add to Selected". Now when the DJ reverts back to the screen showing the track being played. Hit the thumbwheel twice. A menu comes up. Select "Save as Playlist". Name it. Then delete your previous version of the playlist. Your new version will include the track (or tracks) you added.
Okay, that answered your specific question, but let me tell you it is much easier to build and organize, and yes, edit and/or add to your playlists from musicmatch.
After loading musicmatch, up near the top is a button called playlists. On its menu, select Playlist Manager. The window that opens has buttons that will let you create new playlists or modify old ones. You can dock your dj and edit the playlists on it from here or build playlists and then Send them to Portable Device (your DJ) Fairly easy.