They unlock as you level up. Next one you unlock is at level 10. I don't think you get any sort of notification that you unlocked a new stage in World Tour when you reach the appropriate level, so be sure to check back there every 5 levels or so.
Awesome thanks!
Hit level 10, tried the next world Tour stage and got wrecked. Geez, it's like it's zoomed in and there's very little time to react to the incoming notes!