To use the code, you need to post the version of the address from the "embed" code on YouTube.
For example, here is an embed code from YouTube.
<object width="425" height="344"><param name="movie" value="http://www.youtube.com/v/6ow7rqkY-jI&hl=en&fs=1&"></param><param name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/6ow7rqkY-jI&hl=en&fs=1&" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="425" height="344"></embed></object>
I copy the video address out of this, which as you can see is:
http://www.youtube.com/v/6ow7rqkY-jI&hl=en&fs=1&I then put our board's coding onto it:
[flash=425,344]http://www.youtube.com/v/6ow7rqkY-jI&hl=en&fs=1&[/flash]
And presto:
http://www.youtube.com/v/6ow7rqkY-jI&hl=en&fs=1&