Go to 2-1(Green World, Mechanical Garden) and go to the closest Purple Bolt and blast the red switch twice. then make the tall bolt fall. Then get on the purple bar and go off the edge very slowly. Eventually Bomberman will float in midair. This texture is invisible.
Start a new game as Bomberman, then kill yourself with bombs. The Prof will say you need more time at Bomber Mansion. When it asks you to continue, select no and go to the main menu.
Collect all 10 Lightning cards from the Elemental Bombers (including Dark Force) then defeat the final boss in Level 5-4 (War Horse) to unlock the sixth and final world.
To unlock more multiplayer characters, collect the corresponding number of cards listed below:
Hige Bandit 2-Collect all 3 Red Hige Hige cards
Hige Bandit 3-Collect all 3 Blue Hige Hige cards
Hige Bandit 4-Collect all 3 Green Hige Hige cards
Mighty-Collect all 5 Jetters cards
Mujoe-Collect all 3 Yellow Hige Hige cards