In order to get the correct code for embedding YouTube videos you must view the videos in YouTube & get the code there. If you are working through a secondary site, like Florida Memory, click on the YouTube icon on the video and choose to view the video in YouTube.
Once you are in YouTube, click on the "Share" button. Then click on "Embed". A box with "iframe" code will appear. HIghlight and copy all of the text in this box.
In Omeka.net, on your "SimplePage" editor, click "HTML"
This will open a new window that is an HTML editor. Paste the "iframe" code from YouTube there. Click save.