You can check if MMM-Button works by checking the console (not the browser console but where you type in the command to start the MM), it should write "button pressed" when you use the button.
In my JS file, add a Log.info("notification: " + notification + "; payload: " + payload + "; sender: " + sender); in line 26 and see if it comes up and what it shows.
You can check if the cover works by using the developer tools, finding the module and adding the class show-image.
