If it says you can't install it, then you can't install it. It is only related to the boot1 version, not the system menu. However, there was a hackmii installer 1.1 that would give this error by mistake. So be sure you are using hackmii v1.0.
If you don't think it is right, make a nand backup with bootmii ios. Then look at the nand.bin with nandbincheck, used from a shell console. Do-
The first few lines will tell you what version boot1 you have and if it is vulnerable or not.
nandbincheck nand.bin -all -v
giantpune's Nand Tools.zip