I Strongly recommend that you get Nginx configured as the front-end proxy prior to adding the caching. Install the WordPress Nginx proxy cache integrator first & follow the Nginx configuration instructions so that your install is working properly with Nginx and the plugin.
You can install the Network Nginx Proxy Cache Purge plugin in either the /wp-content/mu-plugins directory or
Upload the entire network-nginx-proxy-cache-purge folder to the /wp-content/plugins/ directory
Network activate the plugin in a WordPress network
In single WP, activate the plugin through the 'Plugins' screen