Command to start nodejs? Server.js and app.js missing
If you're downloading the app from github directly, follow these instructions:


Download the archive
Extract to a folder oustide of Drupal's root (this app is separate to your Drupal installation so shouldn't be within)
cd to the folder and run npm install
Run cp nodejs.config.js.example nodejs.config.js, edit the resulting file as required
Run node app.js


Voila, server up and running.

If you're using the npm install drupal-node.js approach, the instructions are a bit unclear. To clarify:


Create a folder outside of the Drupal root
cd to it and run npm install drupal-node.js
Run cp node_modules/drupal-node.js/nodejs.config.js.example nodejs.config.js, edit as required
Run node node_modules/drupal-node.js/app.js to start the server


You should see the same result as the first method.