Am now writing this to the forum on my newly installed 10.6.6
.....and guess what? The Midex 3 doesn't work in this OS X either!!!!!
Driver installation went OK, but no icon in AMS Midi window. However, the unit lights up, although I suspect this is just the USB power, as it even stays lit even when the computer is powered down. (But I did notice that in all versions of Leopard 10.5 there was no USB power to light it, even though I used many other types of USB devices eg memory sticks, external USB-SATA HDD adapters, USB speakers, etc, to name but a few which all worked with no problems).
My conclusion?? This Mac version of the Midex 3/8 driver is faulty, that it doesn't work as advertised with all Mac OS X's from 10.5 to 10.8 (having the phrase "should work" is a bit of a cop-out), that Steinberg know this but refuse to admit it, and instead prefer to hide behind the mask of "it's legacy and therefore unsupported".
I am very disappointed in this whole affair and intend to resign my membership of this forum. I will also be selling all my Steinberg hardware & software, refusing to buy any of it in the future, refusing to recommend or endorse it to any of my friends and musical colleagues, and generally boycotting this company.
Good luck to all of you who struggle to keep going with Steinberg products.