It is no longer available in that form because Stage Traxx no longer plays songs from the iTunes library. You first need to import your songs into Stage Traxx and once they are imported, you can add them to playlists.
There is a shortcut though to allow a similar workflow as in version 2. Just add songs from iTunes to Stage Traxx. If you select more than 10 songs, Stage Traxx will create a new playlist for these songs. You just need to rename that playlist. And this does also work if you already imported these songs. It will detect that the songs are imported but will create the playlist anyway.