When I installed Zend Server CE on OSX 10.6 Snow Leopard the admin interface failed, the following fixed the issue :
1. Download this: Watchdog update
2. Backup watchdog with the command
sudo mv /usr/local/zend/bin/watchdog /usr/local/zend/bin/watchdogOLD
3. Extract the new watchdog to /usr/local/zend/bin/ (I used the go to folder in finder, if you can view hidden files)
4. Run the following command to restart the Zend server
sudo /usr/local/zend/bin/zendctl.sh restart