You simply go the page of your YouTube video that you want to display on this message board, and copy|paste the URL (ie the address) of that page into your post here. So for example, if I paste the following text (I have deliberately misspelled youtube so that the video doesn't just get embedded twice!:
http://www.youtoobe.com/watch?v=uIcdd84MALg
you see this!
The message board figures out that the address points to a youtube video and automatically creates the little video window.