For those of you who don't know, the Steinberg Midex 3 and Midex 8 are USB-powered MIDI interfaces, with 3 MIDI Outs/1 MIDI In and 8 MIDI Outs/8 MIDI Ins respectively.
From your DAW or sequencing software, they allow you to send MIDI data simultaneously with very little latency on all 16 channels of EACH output (3 x 16 = 48 for the M3, 8 x 16 = 128 for the M8). By spreading the load over more than one MIDI cable for large amounts of data, they can help you to avoid what is known as MIDI 'choke'.
These units are now legacy according to Steinberg (which means they are unsupported, especially on Hackintoshes). In the good old days of Classic Macs, you had to FIRST download and install Opcode's OMS 2.3.8 BEFORE installing the Midex driver from the accompanying CD or download. But for us OS Xers, that is no longer necessary and there is a Mac version of the current driver available for free download from their website (steinberg-dot-net). It says "should work" (their words, not mine) for all OS Xs from 10.5 through to 10.8. After installing it, you should be able to see an icon for the M3 or M8 in the MIDI window of the Utility>>Audio Midi Setup (AMS).
It is important to NOT plug in the MIDI interface before or during the installation of the driver, only after you get the "Installation Successful" message.
However, with my Midex 3 I have been experiencing difficulties and inconsistencies with my Hackintosh installs.
Leopard 10.5.3 - after software install, unit does not light up (receiving no USB power although all my other USB devices work OK), and icon is not seen in AMS/MIDI window
Other Leopard distos and versions: the same as 10.5.3
Snow Leopard 10.6.1 & 10.6.2: unit lights up, icon is in AMS and by pressing the Test Seup icon (top right of MIDI window) you can press each of the MIDI out downward-facing arrows and send a MIDI message to each of the ports of the M3 unit.
Leopard 10.6.6i: unit lights up but no icon in AMS.
As of yet, I haven't been able to further test the MIDI Outs as there is no relevant software installed. (Is there a definitive list somewhere of all the DAWs and sequencers available for Macs in terms of which is the lowest OS X version they support??)
Any musicians out there who use their Mac for sequencing, please report back, especially if you have an M3 or M8, or if you use other USB MIDI interfaces.