First, get Eclipse to work well with Drupal. Here are instructions for this first step:
http://drupal.org/node/75242

For Zend debugging, follow the instructions in these urls:

http://slashgeek.com/?p=341

http://echodittolabs.org/blog/2010/08/how-configure-eclipse-pdt-zend-ser...

One edit to the last: the path is
Eclipse->Preferences->PHP->Debug->Installed Debuggers. The author left out '->PHP'.

here's another helpfpul link referred to in a comment on slashgeek:

http://stackoverflow.com/questions/2451671/how-to-install-eclipse-php-de...

I currently have MAMP 1.9.2 running with PHP 5.2.13 and Eclipse 3.4.1.

I installed it in Dec '09 with the help of this url:
http://www.macgeek.co.za/2008/12/01/installing-integrating-and-configuri...

There are also Drupal nodes addressing Eclipse for a mac, with Zend and XDebug:

General Eclipse. Look at plugin configuration to set Eclipse to recognize Drupal file types, to use spaces instead of tabs, and to ensure Unix-friendy files. XDebug described here: http://drupal.org/node/75242
Zend Debugger: http://drupal.org/node/157609

These have been updated since I used them; hopefully for the better!