Here's how to get to it.
1. go to the creator box, and open it.
2. go to the bottom right corner, and click quick play.
3. You then pick any level you've unlocked, and two extra characters from all the game/ ones you've created. (they're extras because you add them to the characters already in the level.
I don't know how many people know this already, so I'm just putting this here anyway.