Your system may need to have the mod updated to be current and/or use current version of USB Loaders. See our Softmod Any Wii Guide for information on both (the mod updates are chapter 2 and the USB loader updates are chapter 3). You'll want to start at chapter 1 to make sure everything is updated.
And we don't recommend using WBFS anymore, FAT32 (is what I prefer) or NTFS formatted hard drives are what's "in". We have guides to both NTFS and FAT32 showing how to fomat the drive and install the games.
Links to all guides mentioned above are in my signature.