If you turn on your Wii WITHOUT an SD card in it, does it flash once, and then flash a couple more times really fast like a second later? If so, bootmii should be properly installed to boot2. If not, you did not install it to boot2.
If it is installed to boot2, make sure your sd card has the bootmii folder still named "bootmii" on the root of the sd card and has 3 files in it. Also check the bootmii.ini file in textpad or something and make sure it isn't set to say:
AUTOBOOT=SYSMENU
BOOTDELAY=0
otherwise it will skip the menu entirely. (If those two lines have a # sign before it, that's ok and it shouldn't autoboot anything.) By default it should say something like:
#AUTOBOOT=SYSMENU
#BOOTDELAY=5
which is fine.
If everything is set up like that and bootmii still does not boot on startup, then try backing up your stuff on your sd card to your pc and then using the sd formatter tool which I believe can be found in the downloads section here to format your sd card and then put only the bootmii folder with those 3 files in it back on the card and try again.
If it STILL doesn't work, your SD card is not compatible with bootmii and you will need a different card that is compatible.