For best results when adding a video from YouTube, use the Embed code rather than the page URL.  To find the Embed code, browse to the video on YouTube, then click the Embed button below the video player:

Make sure all of the checkboxes are UNCHECKED.  Copy the highlighted Embed code (right-click in the box and select Copy).  Then on the 2nd page of the Add Video Wizard select the last option, "Add a video or other media from a piece of html code" and paste (right-click in the box and select Paste) the Embed code in the box. 

 If you leave the Screenshot url blank, the wizard will create one from the video for you.