usbloader = playing game off of a USB hard drive
original disc = you bought a Wii game and are trying to play it
backup disc = you have a copy of a wii game burned onto a DVD-R or DVD+R disc
WiiPower kindly told us that there are only 3 possible explanations of your problem:
1) bad modchip - this is a chip someone (not nintendo) put in your Wii to play backup discs from the disc channel (only works on old Wiis)
2) outdated cIOS - if u followed the modmii guide correctly, this isn't your problem
3) outdated cioscorp - cioscorp allows you to play backup discs from the disc channel without needing a modchip (only works on old Wiis). If you told modmii to update all your IOSs then this definitely isn't the problem. However, I don't know what u told ModMii when it asked about updating your active IOSs. Your custom ModMii guide would be helpful so we know what u did.
So the question remains... how are you trying to play the game? Through the disc channel? Through a usbloader like configurable usb loader? Through a backup disc loader like NeoGamma?
edit: I think I'll let WiiPower field this one... he's all your buddy!