When in the game, when playing in any *chapter (during turn-base mode) of any campaign, press the console (~). Type the following codes:

@superunit(549845) - Black Eagle Tank
@superunit(387546) - Leopard 2 Tank
@superunit(874653) - M1A1 Abrams Tank
@superunit(140329) - Chinese T-98 Tank

After typing the codes, you can then access the super units in your warehouse.

*These cheats do not work during the in-mission game.