Neither documentation nor the tutorial you're following mentions that you need to rename `package.json.sample` and `Gruntfile.js.sample` to `package.json` and `Gruntfile.js` respectively. Rename these two files and run `npm install` and `npm update`.

Make sure you have grunt installed globally as stated in the documentation/tutorial. Don't forget to remove your previously installed `package.json` and `node_modules`.

Please read the documentation: http://devdocs.magento.com/guides/v2.0/frontend-dev-guide/css-topics/css_debug.html

`grunt exec:theme` NOT `grunt:exec`