Sorry, if it sounds hostile but it is not meant that way. It is really explained on the docs (and on the plugins tab on the backend). Third party plugins go to /plugins and officials ones are in /zp-core/zp-extensions.
You should never place plugins in the latter or even touch zp-core stuff so you can just replace the full zp-core on update without worrying to overwrite something.
Also, as mentioned the Piwik plugin is a draft that needs testing. We appreciate that. but there must be any kind of error if you cannot get any connection. The plugin btw, does not get anything, it is meant to collect data for Piwik.