I definitely have the required code they mention in my blog's header tags and have enabled plaintext/HTML as my text-editor. My code matches their format and I stripped all the returns out of the Processing sketch - is there some Tumblr-magic I've neglected, or is there an easier way of doing this?
Same problem! What worked for me was to put a div at the beginning (before canvas...) and a /div at the end (after /script) when you are in the html editor.
Or maybe you could try with this "tumbleryfier" found at http://p5lyon.tumblr.com/ProcessingJSTumblrEn