The httpd.conf directory differs across Linux / Unix variants like CentOS, Mac OS X, Red Hat, Ubuntu and XAMPP Mac.
This lists the httpd.conf location for each operating system:
CentOS 5.2
/etc/httpd/conf/httpd.conf
/etc/httpd/conf/httpd.conf
CentOS 5.3
/etc/httpd/conf/httpd.conf
Mac OSX 10.6 Snow Leopard
/etc/apache2/httpd.conf
Mac OSX 10.7 Lion
/etc/apache2/httpd.conf
Red Hat Enterprise Linux Server 5.6 Tikanga (RHEL)
/etc/httpd/conf/httpd.conf
Ubuntu 7.04 Feisty Fawn
/etc/apache2/httpd.conf
Ubuntu 8.10 Intrepid Ibex
/etc/apache2/httpd.conf
Ubuntu 10.10 Maverick Meerkat
/etc/apache2/httpd.conf
XAMPP on Mac
/Applications/XAMPP/etc/httpd.conf
If your operating system or Apache version is not mentioned above, you can use the find command on Linux, Unix or Mac via SSH/Terminal to find httpd.conf.
find / -name httpd.conf 2>/dev/null