http://httpd.apache.org/docs-2.0/mod/core.html#options
What you're looking for is probably 'Options Indexes'
Place in appropriate <Directory> container in httpd.conf or in .htaccess in the directory you want indexes for (Assuming you have AllowOverride All or AllowOverride Indexes set for that directory)
If directory indexing is turned off, and there is no index.htm (or whatever) file, you will get a 'Forbidden' message.
I'm assuming you're running Apache. 🙂
Also, to go along with stolzy's question... whatever directory it is, make sure the apache user has read and execute (rx) permissions on it.