This gets me closer, just now "MII quits unexpectedly". It also shows me a dump, I can post it here if needed.
For future reference, Open with > Macintosh HD > System > Library > Core Services > Java Web Start in case people have trouble finding it like I did.
EDIT:
After revisiting the dump issue, as long as I don't check the box for "always allow", MII will open right up.