When you get to the game select screen highlight two player contest and press down 5 times to get to music test and another 5 times to get to cheat mode. Once in cheat mode, press these buttons to activate the cheats:
50 Lives: Y, A, Select, A, Down, Left, A, Down. Hard Mode (No Barrels): B, A, Right, Right, A, Left, A, X.
Defeat K.K.Rool in the secret lost world boss to see an extra ending. Additionally, collect all 40 DK coins for an alterate extra ending.
To unlock the Lost World, you must pass the first world. In the all the other worlds, you will start to find a 'Klubba's Koisk' in each world. When you visit one, a Kremling named Klubba will ask you to give him 15 Kremcoins to enter The Lost World. If you do give 15 Kremcoins to Klubba in a particular world, you will unlock one particular level of the Lost World.
Animal Antics Level = Give Klubba 15 Kremcoins in K. Rool Keep
Black Ice Battle Level = Give Klubba 15 Kremcoins in Krem Quay
Fiery Furnace Level = Give Klubba 15 Kremcoins in Gloomy Gulch
Jungle Jinx Level = Give Klubba 15 Kremcoins in Crocodile Cauldron
Klobber Karnage Level = Give Klubba 15 Kremcoins in Krazy Kremland
Krocodile Kore Boss = Beat the game and all 5 Lost World levels.