@MyriamBoure, since www.openfoodnetwork.org is a self-hosted instance of Wordpress, embedding the prezi is just a matter of
- installing the Wordprezi plugin in Wordpress
- going to https://wordprezi.appspot.com/ to generate a shortcode with your prezi URL
- and going into the page or post where you want the prezi to be shown and paste the shortcode
and that’s all. It should not last longer than 10mn to do so. I sure can help but I will need an admin login to wordpress, so it’s probably best to do it on your side.