Look at the video's URL, which should look like: http://www.youtube.com/watch?v=z3gnGW40-Ic
Option 1: Video ID without dimensions
Take the part that I have highlighted in yellow, and place it in a youtube bbcode tag, like this: [youtube]z3gnGW40-Ic[/youtube]
The end result when displayed should be:
Option 2: Video ID with dimensions
On the YouTube page for the video you want to display, if you click on 'embed' on that page, you should see some HTML code that includes the ideal width and height of the vidoeo. If you want to include these so that it is a better size, do as follows:
Option 3: Flash BBCode tag
Option 3 is disabled.