You need the bootmi files on your sd-card. These should be backup on your pc. If not go back into the hackmii installer, select bootmii, there should be an option the prepare sd-card. You don't need to install bootmii, just prepare the card. With those files on your card you should be able to load bootmii.
Not sure about your black screen. Can you access preloader. You didn't do anything wad related did you?