Install the INTL extension with MacPort intalling php5-intl (sudo port install php5-intl), then copying the library to MAMP:

cp /opt/local/lib/php/extensions/no-debug-non-zts-20090626/intl.so /Applications/MAMP/bin/php5.3/lib/php/extensions/no-debug-non-zts-20090626/

Check the correct folder.

Maybe you will need to change the architecture of the MacPort compiler because MAMP is compiled in 32bit

# /opt/local/etc/macports/macports.conf
build_arch i386

Installing the INTL extension on MAMP | Geert Van Damme.