To enable PIWIK site tracking, open the site configuration, go to the Settings tab and click on the Site Identification node. In the Tracking section here you find the checkbox that enables PIWIK tracking.
You'll need the PIWIK Url and SiteId from your PIWIK dashboard to enable the feature on the site.
Important: Remember to add a forward slash (/) at the end of the URL - failing to do so so will cause the tracking to not work.
Remember to click Save to update the changes in the Operations Center.