File > Edit Template > PHP 5.5.3 PHP.ini
In my case this was the version running, so it was the one I had to modify. MAMP let’s you choose among different versions, so take this into account when you modify it.
When you load the PHP.ini in the phpInfo tab of the home you will also see where the PHP.ini file is located
Read the info on this link for further details: http://blog-en.mamp.info/2009/09/increase-php-memory-limit-with-mamp-pro.html