UPDATE 11/30/2013: …
(1) To get a video URL to work with Slideshare, right-click on the video on YouTube, and "Copy video URL." This has worked consistently for me. Also note that Slideshare has changed slightly, but these instructions still work pretty much the same.
(2) In order for PowerPoint hyperlinks to work on a free account, you'll need at least 3 slides inserted before you include one with links (this might not be required on a paid account, but I don't know). The slide with the YouTube video doesn't count in the 3. And be sure to use the Insert, Hyperlink command in PowerPoint as opposed to just typing the URL.
http://bit.ly/jqtCcT Video: How to embed a video into a Slideshare presentation. Peggy Duncan, author, Just Show Me Which Button to Click in PowerPoint 2007 shows you how. First, she uploaded a PowerPoint slide on How to Customize a YouTube Background. Then she added the video on how to do that. She recorded how she added the video to the presentation.