The embed code is found on the host web page. For example, go to this page:
http://youtube.com/watch?v=4P4XJTKzKEU
On the right side of the page you will see two text boxes. One says "URL" and the other says "Embed".
Just copy all the gibberish from the Embed field and paste it in your post and you will get this:
<object width="425" height="355"><param name="movie" value="http://www.youtube.com/v/4P4XJTKzKEU&rel=1"></param><param name="wmode" value="transparent"></param><embed src="http://www.youtube.com/v/4P4XJTKzKEU&rel=1" type="application/x-shockwave-flash" wmode="transparent" width="425" height="355"></embed></object>