Ah, the app in this case is your game Run the export, not the preview from within construct 2. Make sure not to rename the nw.exe or nw.sh (linux), that the dlls are in your games package.nw, the app-id txt in the games main folder and that steam is also running. But this is the very last step to test the greenworks events in your game.
I think you are at the point, where you can start to implement greenworks events in your game. If the greenworks plugin is installed corectly, you should be able to add the object "greenworks" to your game objects - like you would add the gamepad or browser object, to be able to use gamepad or browser events (like if ESC pressed -> close browser tab).
After adding the greenworks object, you can just build regular events which will trigger achievements. The plugin is smart in some way, it will know when an achievement is already activated and do nothing in that case. You should check if greenworks is available as one condition and do the trigger once while true also, so it wont fire every tick when true (which is a good advice for any event that is not required to run every tick). Like that example from my game "Asteroid Deflector XL" on Steam (erm, advertising, *coughing*):
Make sure to use the API-name of the achievement from the Steam app-admin menu to trigger it in the event.
Hope that helps, I struggled with that too for several days. Not to mention the nw.js linux/mac export process with copying files into a fresh nw.js build to get it working... does anyone know if the greenworks support and the linux/mac exports are better supported in construct 3?