not all wii's are able to play backup discs, only the older model dvd-drives can read burned discs.
After using the modmii wizard, if you insert a backup disc it will show up in the disc channel if the Wii is able to play backup discs. Note that you will not be able to actually start the game from the disc channel, only see the game banner load in the disc channel.
If you're Wii isn't able to play backup discs, you're only option is usb-loading (or replacing the dvd-drive\Wii with an older model), and even if you can play backup discs, usb-loading is superior anyways.
If you still want to pursure backup disc loading, after running the ModMii Wizard, you can download\use a usb-loader or neogamma (from ModMii's download page 2), just have modmii save it to your sd card then load it from the homebrew channel (or you can install\use a forwarder channel instead of the HBC)
A few days ago another method of booting backup discs was released but this method uses the disc channel itself. If this interests you, you can find more info here: http://gbatemp.net/topic/321110-poc-boot-neogamma-via-disc-channel/