Make sure you read the comments section for additional info. Specifically, Gregory in comment #4:
"4) Gregory January 11, 2011
Your instructions didn’t work for me, ether in Excel 2010 or 2007, and I tried several times, following the instructions very carefully and to the letter.
What I found out is that, for some odd reason, copying the YouTube URL then adding the ?fs=1&hl=en_US to the end would not work at all for me.
But if I went to the Embed code in YouTube and copied value parameter (minus quotes) and pasted it into the movie property, the movie would play. The Embed code value parameter includes the link and the extra bit you included at the end. No need to copy that as an extra step."