Fine
Take this video for example-
http://www.youtube.com/watch?v=iWFBn8xFy1o
When you watch the video, under the descpription box there should be another box that says "Embed" click it, it gives you a code, like this <object width="480" height="385"><param name="movie" value="http://www.youtube.com/v/iWFBn8xFy1o&hl=en_US&fs=1&"></param><param name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/i=en_US&fs=1&" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="480" height="385"></embed></object>
You look for v/ and then you copy and paste the code on the bottom of "Details", put the name of the video and click save. that should do it.
To get this video in the code would be v/WFBn8xFy1o&hl