To activate  the WooCommerce Product Stock Manager & Notifier pro you just need to download the purchased plugin from here https://multivendorx.com/my-account/api-downloads/ and then go to your wordpress site admin dashboard>>Plugins>>Add new plugin, from here upload the downloaded plugin file. And from the all plugins page activate the plugin. 

Next go to Stock Manager>> License submenu >> put the API details and order id and click on save changes.