Do you have a modded Wii? If so, you will need to burn a disc of the GameCube Operating System (GCOS) in order to boot it.
If you don't have a modded Wii, you will need to get a new one from Datel/Codejunkies. It will act as an GameCube Action Replay, Freeloader, and SD Media Launcher. One downside is that I don't think that you can enter codes on the newest version. I believe that they will replace it for free, although I'm not sure.