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
(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.