Show the Installer Log and report an issue in the Google Groups or Cordova Issue Tracker (JIRA here).
When the installer is launched, Apple-L to show the install log, then select "Show All Logs" from the drop-down list. Run through the installation until you get the error (don't close the installer down yet). Copy and Paste the log.
A previous installer (Cocos2D as an example that does this) installs their own templates using the Root user, causing permissions issues for the PhoneGap installer, which does not need root privileges to install.
Substitute [USERNAME] below for the name of your home account.
The log might show this:
In Terminal, if you do:
It would show for example:
That's the problem, the owner of the folder is root. You'll have to reclaim this for your current user.
Run the installer again, and it should work this time.