OK, I have got it working on our Wordpress site. Not easy.
1st you need to install a plugin called WP Custom HTML Pages. Then you create a basic HTML page with the code from ResDiary pasted into the BODY of it.
2nd you need to install a plugin called iframe (Simple).
3rd Inside your code block you put the iframe shortcode that points to your HTML page that you created before.
Had the same issue Francesca - did you find a solution?
Stand-alone HTML page (to embed to iframe):
script for parent page (insert where you would usually drop the standard embed code):
Hope this helps someone!