This is what I had to do.

Download and install Xcode

Install MacPorts from: http://www.macports.org/install.php

Install the dependences for poclbm. This step may take a long time while it downloads and compiles stuff:

sudo port install py26-pyopencl

Download the sources for m0mchil-poclbm, from https://github.com/m0mchil/poclbm

then open a terminal and extract it, and give it a try.

mkdir Bitcoin
cd Bitcoin
tar zxvf ~/Downloads/m0mchil-poclbm-b981138.tar
cd m0mchil-poclbm-b981138/
python2.6 poclbm.py -d 0 -o yourfavoriteminer -p 8332 -u user@example.com --pass=something

Be aware, that for what ever reason, OS X is FAR slower for mining than Linux or Windows. I run this on a Mac Pro with a pair of ATI Radeon HD 5770s. In OS X I get about 65Mhash/s on one and 95Mhash/s on the other. I installed  Ubuntu 10.10 and boot into it through Bootcamp. There I get 170Mhash/s on each card, over 2x the performance.