PackageManager is MacPython's means for making it easy (easy enough) for users to install and upgrade MacPython extensions. The GUI is buggy. If you have problems, trying running the pimp module, which can be used as a command line tool like so:
[crack:~] bob% python `python -c "import pimp; print pimp.__file__"`
Usage: pimp [options] -s [package ...] List installed status
pimp [options] -l [package ...] Show package information
pimp [options] -i package ... Install packages
pimp -d Dump database to stdout
pimp -V Print version number
Options:
-v Verbose
-f Force installation
-D dir Set destination directory
(default: /System/Library/Frameworks/Python.framework/Versions/2.3/lib/python2.3/site-packages)
-u url URL for database
(default: http://www.python.org/packman/version-0.3/darwin-7.2.0-Power_Macintosh.plist)Feel free to make your own shell script or alias to shorten that command line 
How is it that one is supposed to run those commands (e.g. "pimp -i package")? The best I could come up with was:
python /Library/Frameworks/Python.framework/Versions/2.3/lib/python2.3/plat-mac/pimp.py -i package
but it seems that there should be an easier way.
Make sure to check out PackageManagerRepository, there are alternate databases that have a wealth of additional packages.
How do I get it?
PackageManager comes with a full MacPython distribution. For OS X 10.3 users, get the MacPythonPantherAddons.