I presume you're asking about BootMii.
The HackMii installer won't let you install BootMii as boot2 if your Wii is incompatible. I think Wiis that came with System Menu 3.3 pre-installed have the fixed boot1, so if you got your Wii before June 2008, it should be compatible with BootMii as boot2. If your Wii is not white, or one of the horizontal, GameCube-less Wiis, it definitely can't have BootMii as boot2.