We'll create fresh WordPress site with Processing code directly installed. You have 20 minutes to test the plugin after that site we'll be deleted.
Processing code directly in your web page with shortcode is using this technique :
http://processingjs.org/articles/jsQuickStart.html#processingcodeinwebpage
and adding prismjs (http://www.prismjs.com) for highlight the code
Just add shortcode
[p5js code canvas]
[/p5js]