just quit modmii and try again, if u are having problems, try (re)installing .NET Framework 3.5, or disabling firewall, or stop using a proxy (if applicable).
edit
@Anakinnnn, u can just install my neogamma autoboot mod channel, or my neogamma forwarder channel (or someone else's) and use that instead of the disc channel to launch discs
if u MUST have your backup discs working from the disc channel, just reinstall darkcorp after your done with ModMii, and if u intend 2 use ModMii again on your wii and keep darkcorp intact, first disable ModMii's update active IOSs option.
fyi, u can easily rip all your backup discs to a HDD using wiibackupmanager, it even has a bulk copy feature where it will eject the disc and prompt u for another as many times as u need to. Or, u can always re-rip them using your usb loader, heck, u can even use the usb loader to just launch a backup disc