I'm not getting anything to show up either. I even tried turning off all the custom vars. I looked at the source code after pulling up index.php and it looks like it should work. And Piwik definitely works since it's monitoring several other sites fine.
ETA: I ended up disabling the add-on and just manually pasting my tracking code into the footer template.
Piwik is a downloadable, open source (GPL licensed) real time web analytics software program. It provides you with detailed reports on your website visitors: the search engines and keywords they used, the language they speak, your popular pages… and so much more.
Piwik aims to be an open source alternative to Google Analytics, and is already used on more than 200,000 websites.
Then it was time, that you recognized it. Piwik is a really cool software. You can host it on your own, and did not give away all the data you collected. That spents more trust to your users. And it is fast! I host my Piwik on another default WebPackage for a few coins the month and it's stable.