The IAC Driver is an alternative to Rewire or RAX; it creates a virtual MIDI 'bus' that allows you to input/output through the driver. All you have to do is find another software that will host your synth.

Try this:
- Go to the Audio/Midi Setup application, in the Midi Devices tab
- Select the IAC Driver and press Command-I (or menu MIDI->Show Device Info)
- Put a check-mark in the "Device is online" box
- Under the Ports list, you'll notice there's already one called Bus 1. Use the Plus button to add another Port, it will likely be called "IAC Bus 2".
- Exit out of that window.

Now open BIAB2009Mac
- If you haven't done so already, open menu "MIDI->Select MIDI Driver or Apple DLS Synth".
- Select MacOS X CoreMIDI
This will bring up the Select Ports/Instruments dialog. (If you already had the CoreMIDI driver selected, use the menu "MIDI->CoreMIDI (Choose Ports) to bring this window up.
- Now in each port you'll find the two IAC busses (in addition to any other midi interface you may already have.)
- Try choosing the "IAC Bus 2" port for everything (in and out)
- Now go into your synth host and choose the same IAC bus for your synth. If your software is capable of seeing MIDI hardware (i.e. through a USB MIDI interface), it can also see the IAC Driver busses.