The Vuex library is included the same way as Vue itself. You can either use a hosted version by using the previously mentioned unpkg service (https://unpkg.com/vuex) or you can download the JavaScript library from their https://github.com/vuejs/vuex.
Add a new <script> block to the bottom of your HTML file. Ensure the Vuex library is included after your vue.js library but before your application JavaScript:
<script type="text/javascript" src="js/vue.js"></script>
<script type="text/javascript" src="js/vuex.js"></script>
<script type="text/javascript" src="js/dropbox.js"></script>
<script type="text/javascript" src="js/app.js"></script>
If you are deploying an app with several JavaScript files, it is worth investigating...
