Most will want to simply download the .dmg of the executable from the psychopy website.
Developers can install a local version of the repository using github. See the instructions at the online psychopy documentation "for developers" section.
This requires installation of the python library dependencies. On OSX, rather than installing all the dependencies one by one, you can download and run the installer for the Enthought Python Distribution. At the time of this writing (Feb 2012), it's available for free to academics. Once that is done, execute psychopy in the terminal with something like
Rather than type that every time, create an alias, maybe called 'pp', by adding a line to your ~/.profile like this:
alias pp='python /Users/alex/Documents/softwareStatsEquipment/programming_psychophysics/psychopy/psychopy/app/psychopyApp.py'
Psychopy will only run if the location of the dependencies is in your PYTHONPATH. Most of these will have been added by the Enthought Python Distrubtion installer, but not psychopy itself. To your ~/.profile file, add the path of psychopy itself with a line like: