click the youtube icon in the top right corner of the advanced reply box, not the quick reply one. [YOUTUBE*][/YOUTUBE*] will appear in ur reply, or u can just type those tags out, but leave out the *.
Copy the url from the youtube address bar after where it says - watch?v=
So: h*tp://www.youtube.com/watch?v=muLIPWjks_M&feature=channel_page
Becomes: muLIPWjks_M&feature=channel_page
And paste it inbetween the tags that appear in ur reply when u click the youtube icon. And you're done
[YOUTUBE]muLIPWjks_M&feature=channel_page[/YOUTUBE]
Make sense? Good, now u better not post a shit video.