this file is normally within the conf/apache directory of your webserver, more often than not though in a shared hosting environment the providers don't allow access to this file.
run the phpinfo script I gave earlier and this will tell you if this is enabled as it may not be the httpd.conf file that is causing the problem. if you google setup mod_rewrite you will find lots of info.
One other thing to check is if the .htaccess file has been created, this is created by the installer and contains the rules. Sometimes I have to create a blank version with write permission before running the installer.
If you setup SS on your pc then you will see the files that are being mentioned and see what they contain, this may help you more.