Success came by following a manual process similar to that copied from the Neo Geek article below, Setting up MAMP, PEAR, and Headress:

PEAR Installation

1. After locating the framework you want on pear.php.net, download the latest version (or the version you need for testing) from the "Download" tab on that framework's main page.
2. Unzip the downloaded file.
3. Open the folder that was extracted to find another similarly named folder and a file named package.xml.
4. Copy just the folder to Applications/MAMP/bin/php5/lib/php/. (I copied just the contents of that folder to the HTML folder in my example below.)

Notes on Step 4: 1) If you are using MAMP Pro, you still install your pear packages in the MAMP path shown. 2) There is a PEAR folder in Applications/MAMP/bin/php5/lib/php/ PEAR packages you add manually go in Applications/MAMP/bin/php5/lib/php/ not in Applications/MAMP/bin/php5/lib/php/PEAR/

I didn't want to change include statements in the scripts I was testing. Instead I created a folder in Applications/MAMP/bin/php5/lib/php/ for the top level package name, HTML in this case, and copied the files in the extracted package folder into the HTML folder. So for HTML_Common there is a file named Common.php in my Applications/MAMP/bin/php5/lib/php/HTML/ folder. If this path structure doesn't work for you, refer to the error messages PHP is generating to help you figure out where to locate packages.