Donkey Kong Country Returns : Level 3-2 Button Bash (Mirror Mode)

To unlock Mirror Mode, you need to collect all the K-O-N-G letters in each stage. Once you collect all the KONG letters in each area, a secret level (K) will unlock. After completing all K levels and beating the game, a secret temple opens up. After completing that level, you receive a message explaining you unlocked Mirror Mode. It gives you a basic rundown of how the mode is played and says, "good luck" or something.