To get the Nintendo Tanks, you need to enter a code in various places.

Tank	How to unlock
Bowser Tank	At Forewood Forest, press A B A Y Y L R START.
Mario Tank	At Ducktor Zids Shop, press A B Y Y X L R L L R X Y Y B A B B B A Y X X Y.
No Tank (also refered to as Invisible Tank)	At Ducktor Zids Shop, mash L and R really fast.