Please follow this installation process: General installation process
The archive file content has to be uploaded to: app/code/Magetrend/StockNotification/
The extension is not working without the license key. If you don't have it or don't know yours, please follow this article: http://www.magetrend.com/magento-2-out-of-stock-notification/user-guide/licensing.
To install the license key, please go to:
Stores >> Configuration >> Magetrend Extensions >>Out of Stock Notification>> Information & License
Enter the license key, and press "Save Config" button.
Note: please use development license key if have installed extension not in product store.