Zend Server 5.6 PHP 5.3, is configured to run with the Apache server from your CentOS packages (httpd),
so if it was not installed prior to Zend Server, it will be installed as a dependency.
Are you looking to set up Apache config, or did you get any error which mentions the path '/usr/local/zend/apache2' ?
Common paths are:
Stop/Start/Restart/Status can be used with Zend Control script:
# /usr/local/zend/bin/zendctl.sh [start|stop|restart|status]-apache (use one command at a time :)
Zend Support Team