For some reason, PEAR is not installed on Mountain Lion. But fortunately. an installation PHP Archive is available to install it. It’s located in /usr/lib/php. It is thus very easy to install it using:
sudo php /usr/lib/php/install-pear-nozlib.phar
Enter the password and it gets installed in a second !
You can check the installation by running pear version:
$ pear version PEAR Version: 1.9.4 PHP Version: 5.3.15 Zend Engine Version: 2.3.0 Running on: Darwin NEWMACBOOKPRO.local 12.4.0 Darwin Kernel Version 12.4.0: Wed May 1 17:57:12 PDT 2013; root:xnu-2050.24.15~1/RELEASE_X86_64 x86_64
Of course an alternative is to install MAMP or XAMPP which come with PEAR installed.