You have to install Piwik (www.piwik.org) first before using this plug-in. In the params section you just enter the tracking code piwik is given you.
This plug-in is useful if you are switching templates within your site, the tracking code will be written in the current index.php.
This plug-in comes with german and english language files
Hint: when I installed the plug-in, it appeared as plg_system_piwik in the list (I was looking for nfrPiwik or Piwik). Once I found it, easy to paste Piwik script and activate.
Thanks for the hint; I didn't noticed before; its solved by now with version 3.0.2
5 Stars for sure!