I have recently got a mac an am not used to developing on a mac at all.
I have MAMP Pro 1.9.6.1. I did a locate on php.ini and got this:
It depends on which version of PHP your MAMP is using. You can find it out on: /Applications/MAMP/conf/apache/httpd.conf looking for the configured php5_module.
After that, as someone said before, you have to go to the bin folder. There you'll find a conf folder with a php.ini inside.
example: /Applications/MAMP/bin/php/php5.4.10/conf
Leo