Okay, still no success getting the Dice MIDI port activated.
Uninstalled the Dice 3.5.6 firewire driver with the deinstaller that came with the package. Then installed it again, followed by a restart. Firewire is fine, but no MIDI port.
Scanned through the Harmonizer Setup menu pages, but can't find anything that looks relevant. I think the original 3.5.5 Dice driver simply came with the MIDI port enabled, no options.
This OSX system was originally an OSX 10.6 Snow Leopard installation when I installed the original 3.5.5 Dice driver. Then installed Lion on top of that (without uninstalling anything). Then installed the beta 3.5.6 Dice driver on top of that. Pretty standard sequence of things I guess.
Running out of ideas. Maybe it's simply a bug in the beta Dice.