In order to post an embedded YouTube video you have to switch to the PlainText text format before you paste in the <iFrame> code you get from YouTube. Then you MUST switch back to the Full HTML text format before saving. The text format selection box is found below the comment box.
The text editors that are used by the website tend to manipulate the HTML markup language preventing it from working, so pasting it while in plaintext mode avoids any manipulation.
