Select Single Player mode, go to the level selection screen, and choose the Contra level. Then, press UP(2), DOWN(2), LEFT, RIGHT, LEFT, RIGHT, CIRCLE, X at the "Enter Secret Password" prompt.

ACHIEVEMENT - HOW TO UNLOCK
Rumble in the Pond! (10 points) - Won a single player Battle Royale!
Twin Frogs (10 points) - Got a Twin Frog pair Home!
Challenged (50 points) - Completed every challenge!
Frogger in Love (10 points) - Rescued a Lady Frog in Classic!
Highway Crossing Frog (20 points) - Completed the fifth difficulty level in Classic Frogger!
Nostalgia (10 points) - Got a Frog Home in Classic Frogger!
The Devoted Artist! (20 points) - Cleared all the Paint Patterns!
Go! Frog! Go! (10 points) - Used a Fly power up against only CPU!
Twin Stick (10 points) - Tried both Hyper and Retro controls for Classic!
Tongue Tiled! (10 points) - Won a single player match in Tile Capture!
Around the World (20 points) - Played every game mode!
Greasy Joystick (20 points) - Obtained a score of over 100,000 in Classic Frogger!