I'm not too familiar with the MAMP setup for php but these settings should be defined and set in your php.ini file. If you create a php info page you should be able to find the loaded ini file and adjust it there.
To make a phpinfo page add this line of code to a new php page or an existing one:
<?php phpinfo(); ?>
In here look for the loaded configuration file setting at the top of the listing. This is where your ini file should be, if you can adjust this file you should be able to turn the display errors setting to on, and set the error reporting to E_ALL.