To insert  html, widgets, youtube, and other videos:

First, copy the embed code (adjusting height and width as needed):

<object width="425" height="344"><param name="movie" value="http://www.youtube.com/v/LlfnRsr6hU4&hl=en&fs=1&"></param><param name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/LlfnRsr6hU4&hl=en&fs=1&" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="425" height="344"></embed></object>

You can insert the video into articles by going to articles "text" area, and hitting the "source" button, and then inserting this embed code right into the text field.

Hit "source" again to close this area and then be sure to "save".