WordPress.org

The easiest way to install the plugin is to use the plugin installer in WordPress.

If you want to do it manually:

1. Upload the processingjs directory to the /wp-content/plugins/ directory
2. Activate the plugin through the 'Plugins' menu in WordPress

Once activated, you'll notice a 'processing' button when you're writing or editing a post in HTML mode. Clicking it will insert a very simple Processing sketch into your post. You can use it as a template or write your own from scratch. You'll find examples (with code) of what can be done here: Processing JS Demos.