I had the same problem (a year after the last answers here). I managed to install the extension under MAMP in the following guide: http://sunny.chotai.uk.com/?p=2
Basic steps: 1. install php54-intl using MacPorts
sudo port install php54-intl
2. copy the intl.so file to the php MAMP folder by doing:
cp /opt/local/lib/php54/extensions/no-debug-non-zts-20100525/intl.so /Applications/MAMP/bin/php/php5.4.4/lib/php/extensions/no-debug-non-zts-20100525
3. Enable the extension in php.ini by adding the following line to the end of the file:
extension=intl.so
4. Reboot the servers and enjoy
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