The default embedding code provided by YouTube is now with the iframe tag, not the object or embed tag.
The iframe tag to embed videos works in a label or page in Moodle, but not in the wiki.
After following the above suggestions from John, I embedded a YouTube video using the object tag.
For example:
<object data="
" type="application/x-shockwave-flash" height="400" width="600"><param name="allowScriptAccess" value="never" /><param name="allowNetworking" value="internal" /><param name="allowFullScreen" value="true" /></object>
Then it worked for me
To learn more about these tags, see http://www.w3schools.com/html/html_youtube.asp