hey thomas,
unfortunately there are quite a number of reasons why embedding of html might not be working. the 2 major one's would be that your joomla editor is preventing things like "script, html, head, body, iframe ..." in the settings or even more likely that the joomla ACL setting is restricting it. if you head over to your joomla configuration at the tab "text filters" and the usergroup trying to insert html has a value other than "no filtering" it's not going to work.

if both are configured in a way that using html is allowed please provide us with your site details and ftp access so that we can investigate this issue directly on your site.

hope that helps but if you have any questions feel free to get back to us at any time. have a nice day