The easiest way is to use Magento Connect. If you need to do manual install, all you need to do is copy over the app folder to your Magento one.
##In Magento Open up the Piwik Analytics Control tab, enter the info and voila, all eCommerce data will be in your Piwik install.
This works on Magento 1.x series
Update on the status of this plugin: http://www.statstory.com/piwik-ecommerce-extension-for-magento-update/