Just select 'varnish' for 'webcache' in 'switch programs'.
Varnish settings is basic. Still trouble with ssl.
For high traffic website, better choose nginx-proxy (already include microcache) or hiawatha (need add microcache code inside index.php) with install/enable their php apc or xcache, enable/install mod-pagespeed for apache and enable cache in website application (like hypercache in wordpress).
