Hey there -- I wrote that article. I guess this is what fame feels like :P
What exact issue are you having? From looking at your code snippet above, your path is wrong for your jQuery plugin include.
Assuming you uploaded it in the PUBLIC folder in Content Manager:
CPGuest > Config > Content Manager > Public folder > Upload > jquery.qrcode.min.js file
Then your include should be:
<script type="text/javascript" src="public/jquery.qrcode.min.js"></script>
What is your exact issue?