Things do change behind the scenes in Nabble from time to time. There's been a more recent discussion on the Premium Support forum on this topic. My current understanding is that providing you are only trying to embed videos from the currently supported sites, the EMBED button mentions "Videos from Youtube, Vimeo and LiveLeak." then things should work.
With videos from other sites you need to check the box "Message is in HTML format" (and write your message using appropriate tags for paragraphs, etc and pasting in the < iframe > code from the site) and then Nabble code that identifies a video is more forgiving.
See:
http://support.nabble.com/Embedding-Video-Issues-tp6879107p7580865.htmlIf it still doesn't work you might try posting the video here, or, at least, the code you are pasting.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.