**NOTE** The following solution doesn't work for people who are running Protools as Protools doesn't allow the MBOX to be used as a multi-client sound card (unlike every other application). But it works if Protools is closed.
Here's what I do to use the Mbox as my sound card for OSX (as well as for my other applications such as Logic and Ableton Live):
1. Download and install Soundflower (by Cycling '76) which acts as an internal sound card (and will show up in OSX and all your applications):
http://code.google.com/p/soundflower/downloads/list
2. Download and install Pure Data, a free open source music programming software, you can use this to patch Soundflower to your Mbox:
http://puredata.info/downloads
3. This is the patch we'll write for puredata:
Open Pure Data, select FILE/NEW, in the new window select PUT/OBJECT, and write "adc~" (minus quotation marks) in the object box. select PUT/OBJECT again and write "dac~" in the object box. place adc~ above dac~ and connect the left output from adc~ to the left input of dac~, and do the same with right output and input. select PUT/OBJECT, and write "loadbang" in the object. select PUT/MESSAGE and write ";[RETURN]pd dsp 1" in the message box ([RETURN] means press the RETURN button on your keyboard). Place [loadbang] above the message object and connect the output from loadbang to the input of the message box. This part turns on the "Compute Audio" button (located in the main window) automatically, when you load this patch in the future. For now, select the "Compute Audio" in the main window manually.
4. Go to PREFERENCES/AUDIO SETTINGS in Pure Data, select Soundflower (2ch) for input and Digidesign for output. click OK
5. Go to PREFERENCES/STARTUP, click APPLY, then click OK. (this saves the input and output for when you open the program again for some reason).
6. Save the file to your desktop (for convenience) and now when you click on the file, puredata connects OSX to your MBOX! just hide puredata, so it's running in the background. You might have to fiddle with the "delay" value in the audio settings window in PureData, but somewhere between 13 and 20 msec should work.
*there's a little bug in PureData with SnowLeopard, when you click the "compute Audio" on, then off, then on again, PureData crashes, but other than that, I've had no problems, and everything works seamlessly.
I know this seems really complicated, but I like using puredata especially for simple and elegant little workarounds such as this.
Die hard MBOX!!!!!