These are the best guidelines I have found and mashed together recently for installing and configuring PEAR and PHPUnit for use with MAMP.

Firstly tell PEAR to auto discover:

sudo /Applications/MAMP/bin/php/php5.4.4/bin/pear config-set auto_discover 1

Secondly install the PHPUnit PEAR package:

sudo /Applications/MAMP/bin/php/php5.4.4/bin/pear install pear.phpunit.de/PHPUnit

Thirdly link it to your profile:

sudo ln -s /Applications/MAMP/bin/php/php5.4.4/bin/phpunit /usr/local/bin/phpunit

Lastly test it:

phpunit --version

Should output:
PHPUnit 3.7.10 by Sebastian Bergmann.

All done!