Ok, download the bootmii folder from here:
Unzip it, so that you get a folder called bootmii with three files inside of it.
Place that bootmii folder onto the root of your sd card, also place the nand.bin on the root of your sd card.
After that, insert the sd card into the wii before turning it on. Bootmii should appear, navigate with the Power/Reset/Eject buttons on your console, go to the icon with the gears and select restore nand.