Is there a bootmii folder on the SD card? If not, run hackmii installer again and choose "Prepare SD Card" to create a new one.
If you have a bootmii folder already, the SD card is incompatible. Get a different SD card. I use both a Kingston 2GB MicroSD, and a 1GB Sandisk (Make sure to move the bootmii folder to your new card).
Also, you don't need to unplug your wii when it freezes. Just hold the power button down until it turns off (~5 seconds).