I had the same problem, until I remembered reading the article on the Wiibrew Wiki. See picture below!
You also have to add the WiiMC homebrew channel app to the apps folder on your SD card. This also works if you're running from a reader, hence "USB". After doing this, it should work!
You also have to add the WiiMC homebrew channel app to the apps folder on your SD card. This also works if you're running from a reader, hence "USB". After doing this, it should work!