This usually occurs when the URL being pasted in is not a full direct link to the YouTube video.  Please verify that:

• The link is the full URL, not a shortened version
• The link is the URL and not the embed code provided by YouTube for the video. The embed code is unnecessary as SlideRocket takes care of the embedding
• The link is the direct, published link to the YouTube video and not a link to a YouTube user page or private video