The Konami code:
↑ ↑ ↓ ↓ ← → ← → B A Start
or for 2-player
↑ ↑ ↓ ↓ ← → ← → B A Select Start
(technically, the "Start" and "Select, Start" are not part of the code, but are necessary to start the game)
There are 6 ending in total: 4 "regular" endings that you get for following 3 route splits (one at start and then one more split on each branch), a "bad ending" if you agree to join Colonel Bahamut when offered this choice before his boss battle and secret "prehistoric age" ending, if you climb up the wall before Noiman Cascade (the hacker)'s boss room.
None of 2 special endings add additional full missions. Bad ending just ends game right after you choose to join. Prehistoric age, however, gives you several bonus boss fights in quick succession, much like Noimans's fight would.
Best Answer
The Konami Code should be entered in the main menu, not in a level.