When you click "Install new addon" you should then see a file picker. You should pick the .c3addon file for Greenworks.
To install an addon from a .c3addon file, first select Menu►View►Addon manager to open the Addon Manager. In this dialog, click Install new addon... and choose the .c3addon file. Construct 3 will prompt to confirm installation of the addon. If you confirm the install, you must restart Construct 3 before the addon is available. In the browser you can just press the Reload button.
Once it's installed and you've reloaded C3, like all installed plugin, it appears as a separate "Greenworks" plugin. Addons cannot (currently) add actions, conditions or expressions to other addons.