This is an old post but it might be worth noting the reason why Prezi won't work for an assignment submissions. The information below will also apply to other iframe sources, e.g. YouTube and Slideshare.
"For security reasons, moodle just does not allow you to use iframes everywhere. In general, iframe code is not allowed in Moodle activities to which all class members can contribute, including Assignment submissions, Wiki, Forum, Database, Glossary, and Blog activities. In these activities iframe-embedded video displays in the HTML editor, but will be stripped out when posts or submissions are saved."
Taken from https://docs.moodle.org/29/en/Iframe.
Cheers
Richard