to boot preloader, turn on wii while holding the reset button.
For bootmii, add bootmii files to a compatable sd card and turn on the wii. Alternatively, if you haven't got a complete brick and can boot preloader, boot preloader and load the hombrew channel, then press the home button and select bootmii ios (or whatever the option is)
Have you installed bootmii as cios or boot2?
Also, how did the wii brick?