These are referred to as "Mystery Quests" by the game. When you tap on the icon for the plane (or train, boat, etc) that is just past the last level you've cleared, you should see a menu pop up.
On this menu you should have at least two options. One will be to pay to unlock the next episode, and the other will be to connect to Facebook and ask your friends for help.
The third option only appears if you are playing on a mobile device (iPhone, iPad, Android, etc) and not logged into Facebook inside Candy Crush Saga. and this This is the option to play a Mystery Quest.
If you don't see this third option, log out of Facebook by:
- From the "Map" screen, tap the arrow icon in the lower left. This brings up a small menu.
- Tap the red "Door" icon, which will take you back to the main menu.
- Tap the arrow icon in the lower left again to bring up the small menu.
- Tap the green "Gear" icon to bring up another menu.
- Scroll this menu down to the bottom and tap the Facebook icon which should say "Log out"
Then you should be able to go back into the game, and tap the plane/train/boat/etc icon and see the Mystery Quest option.