Many Bothans died to bring us this information (or at least my patience died a small death). Stashing this here for next time and/or to help someone else hopefully.
brew install icu4c
Next download PHP source that matches your MAMP version.
mv ~/Downloads/php-5.3.14 include/php
pecl install intl
Finally, add "extension=intl.so" to your php.ini and restart Apache