There has been a general trend to move to using forwarder channels instead of channels with the actual app embedded in it. Forwarder channels don't need to be changed to update the app, they take up less space on your Wii, and you can use them even if you don't have an SD card, assuming your hard drive is FAT32 (or has a FAT32 partition). ModMii has a USB-Loader forwarder channel ("USBX" on download page 2), which is downloaded as part of the ModMii Wizard's 'extra brick protection' (so u may have it installed already). If you copy the USB-Loader setup files to your FAT32 HDD or to your SD card, then this forwarder channel will work for you, otherwise it will return you to the main wii menu.
If your hard drive isn't fat32, then u can't make a forwarder channel work (without an sd card), so yes, u would need to install an updated channel wad. But I would still recommend using a forwarder channel to launch the app off of a FAT32 hard drive (or SD card).