I still don’t understand your issue.
First you tell about the error message “undefined function is_site_admin()”. Either you use the plugin in WPMU/multisite mode while WordPress isn’t or you are using an old WordPress version. “is_site_admin” should be available at each up-to-date WordPress installation running in multisite mode.
Second you ask where to find URL and auth code. You have to get it from your Piwik installation. If you don’t know anything about Piwik first get it from https://piwik.org/ – a running Piwik installation is required to run WP-Piwik.
Have a look at the Piwik demo: https://demo.piwik.org/
If this was your own Piwik installation, you would use the URL https://demo.piwik.org/ and the auth token on https://demo.piwik.org/index.php?module=API&action=listAllAPI&idSite=1&period=day&date=yesterday or an user’s auth token.