Super easy and you do not really need to know how to use dashcode -
1) open dashcode and select "custom"
2) under test and share select 10.4.0 backward-compatibility (under options)
3) name the widget Youtube embed
4) click "save to disk"
5) save the widget to your desktop
6) right click on your widget and select show package contents
7) open main.html with your text editor and replace all the html code in main.html with:
<center><font color="#F7F7F7" size="4"><b>YouTube Video Embed Beta</B></font></center>
<center><iframe class="youtube-player" type="text/html" width="280" height="225" src="http://www.youtube.com/embed/6NFrHD4-wY0" frameborder="0">
</iframe></center>
save the file and close the widget folder - drag and drop into your iBooks HTML container, enjoy ;3
I have HTML5 CSS3 Canvas Javascript animations, complex calc javascript and a whole host of other things using this simple process of modifying a widgets base files. Its really pretty slick.
Note: you can set the width/height in the info.hlist file/
Enjoy!