To use this extension, you have to add
the following three lines to your LocalSettings.php

include("extensions/GraphViz/GraphViz.php");
$wgGraphVizSettings->execPath = "/usr/bin/";
$wgGraphVizSettings->mscgenPath = "/usr/bin/";

Then run mediawiki-addGraphvizextensionToInstances as root
to enable the extension in your mediawiki instances.