open "/private/etc/apache2/httpd.conf" in an editor. Scroll down until you find this part:
<Directory "/Library/WebServer/Documents">
Inside there you'll need to add "Indexes" to your "Options". For me, this meant changing this line:
Options FollowSymLinks Multiviews
To:
Options Indexes FollowSymLinks Multiviews
Then restart apache:
apachectl restart