This method of GameCube backup loading is no longer recommended. Please see Nintendont for the better method.
Just like Wii games use IOS to launch, the GameCube games use the MIOS instead. This is a special kind of IOS which loads only GameCube games.
If we want to enable GameCube backup support, we will thus need to install a cMIOS.
If you use this method, you can not use DIOS-MIOS or DIOS-MIOS Lite.
This does not work on newer Wii models!
If your Wii is not White or not old enough, it will not read backup discs!
1. Run the NUS Downloader. Enable Pack Wad.
2. Download the MIOS patcher package. Put the cMIOSWiigator folder of the zip in the apps folder of the SD card.
3. Put the SD card in the Wii. Go to the Homebrew Channel and run the MIOS patcher. (It is called "Installer for WiiGator's cMIOS").
4. It will ask for an IOS to use. Change the IOS to IOS249 and press A.
5. It will give you a warning. Wait for the Press 1 message to appear, then press 1. (Doing so before the message appears will quit the installer)
6. Now you basically all you have to do is press A every time:
You should not need to change any other settings.
Once you are done, the cMIOS will be installed. You can now load GameCube games from backup discs!