A PlaylistID is basically a number associated to a playlist, so that no matter what you rename the playlist, there is a way of calling it. MC assigns the playlistid when you create a playlist. I'm not sure that it's possible to manually change it.
It used to be that when you created a smartlist based on a playlist, it would show you the playlistid. That wasn't very user friendly because if you wanted to easily see what playlists a smartlist was based upon, you couldn't really tell, because all you'd see was a bunch of meaningless (to you) numbers.
There is still a way of getting the playlistid, though.
Create a smartlist.
Add Rule -> Playlist and then choose the playlist you're interested in.
This rule will show up with the actual name of the playlist, but if you right click on that rule and choose "Edit by typing" it will change and show you the playlistid.
I'm actually kind of suprised to see them using the playlistid in the SDK. I would think that any kind of plugin that calls a playlist would be be a one user plugin, since playlistid's would be unique from one machine to the next.