First, please ensure you have a current and complete backup of your disks and your data. If you should make a mistake or if the installation should tip over and scribble on something, then that backup (or a reinstallation) is your best and easiest path back to an operational system.
As for your question around loading PEAR/PECL tools onto 10.6 Snow Leopard, Google finds [this|http://clickontyler.com/blog/2008/01/how-to-install-pear-in-mac-os-x-leop ard> and [this|http://stackoverflow.com/questions/628838/how-to-set-up-pear-on-mac-os-x- 10-5-leopard] and [this|http://discussions.apple.com/thread.jspa?threadID=2602597&tstart=105], and one or more of these postings might get you going here.
(One other thing that's happened with Snow Leopard and Snow Leopard Server was the upgrade to php 5.3 and more recently to 5.3.3. Based on various postings, PEAR tends to toss deprecated errors because of this; php is newer than the code. This might have been fixed in a newer PEAR.)
The other piece you'll need to load is mcrypt. [Here is a recent thread on Magento and php and mcrypt|http://discussions.apple.com/thread.jspa?threadID=2722257].
There are various related discussions of installing Magento on Mac OS X and Mac OS X Server over in the Magento forums, as well.
