First, I would carefully check the HTML snippet you pasted into your project: I've seen some posts where people inadvertently forgot a <, / or > in the surrounding HTML.
Second, in trying your URL in a sample project on my setup, I see that YouTube is retuning a Flash object. This obviously won't work on iOS. If you can get the raw H.264 URL, ending in ".mp4", you are much more likely to succeed. Once you have the MP4 URL, use a Video object, open the Inspector pane, and set the Video property to the raw URL. (As an example URL, I used a clip from the owner of the one you used: http://www.kidstv123.com/video/MP4%20Phonics%20Song%20Clip.mp4)
If you enable Preview > Show Web Inspector When Previewing, the console tab may show you some additional information.
Also, what is crashing? iAd Producer? Safari? One of the iOS test apps? What version of iAd Producer are you running?