Download the usbloader gx dol, put it to the the root of your sd-card, put wadmanager in it's standared apps/wadmanager/boot.dol
Install the usbloader gx boot.dol into preloader. Then boot the installed file, have a usb drive pluged into the outer usbport, exit usbloader to hbc, run wadmanager from there. If that does not work put the hackmii installer on the root of your sd-card and load it into preloader, then exit the installer, it should exit to hbc, run wadmanager.
If wadmanager still fails, try cboot2 and load wadmanger from bootmii, load bootmii from hbc.