Have you installed and started the Virtualization Server (used to preview content within a sandbox?).
You will need to have this separate tomcat instance running for purposes of preview. Also, to enable preview you will need to either have an entry in your hosts file to match Virtualization Server styles URLs to remap to localhost, or use either your own DNS server or a hosted DNS server that we provide to auto-map virt server URLs back to your installed Alfresco instance. That does require Internet access, however.
You can read more about this on the Virt Server FAQ on our wiki.
Kevin