@ Michael
Switch to HTML mode and type in [ youtube ] [ /youtube ]
Delete the spaces between the brackets and youtube and /youtube. Between the inner ] [ brackets you need to put in the numbers/letters combo after the v= part of the youtube url. So for the video I posted above, the url is http://www.youtube.com/watch?v=HeytH_GN08o&list=UU5elZt69ka5BM1r-XFeENbQ&index=0&feature=plcp. You just need the HeytH_GN08o&list=UU5elZt69ka5BM1r-XFeENbQ part in between the brackets. Don't type the full url. Also, ignore any words that come after the code bit like "feature" or "index".
So the end result should look like this [ youtube ]youtube video code[ /youtube ]
and remember without the spaces.