Change the Name of a Hero to one of the following. This can be done from Barrack under View Soldiers, clicking a Soldier, then going to customize. It can also be done from the customize menu on the deploy screen. Note, using one of these heroes will disable achievements.
Code - Result:
Joe Kelly - Unlock Joe Kelly - Heavy
Ken Levine - Unlock Ken Levine - Sniper
Otto Zander - Unlock Otto Zander - Assault
Sid Meier - Unlock Sid Meier- Support