Superman first appears as a playable character in story level 5: "Chemical Crisis", about halfway into the level. Once you can play as Superman, press the

button (or cycle with

or

(the shoulder buttons)) to choose him.
As Superman, "double jump" (Press

twice) to become airborne. Then move around with the left stick. Use

to increase elevation, and

to drop. You'll get the achievement in less time than it took to read this.