For piwik working correct, you have to edit the javascript-code it gives you by hand. Also, your hda has to be reachable from the websites you are tracking, like - if you want to track your website example.com, you need to forward HTTP-Traffic to your hda, regulary thats Port 80. Inside the javascript it gives you, you need to specify the url to your hda-piwik-installation then - like

<p style="color:#F00; font-weight:bold;" >This may open a big security-hole for your system. So - if you're not familiar with firewalls and security like setting up .htaccess and .htpasswd for your hda, you should consider to install the piwik at an server outside your LAN.</p>