First you copy the embed code from the video...
<object width="640" height="385"><param name="movie" value="http://www.youtube.com/v/vt2v3tpySSk&hl=en_US&fs=1"></param><param name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/vt2v3tpySSk&hl=en_US&fs=1" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="640" height="385"></embed></object>
Paste it on the reply, and then highlight the whole code and click the youtube button on the top tool bar.
[YT]<object width="640" height="385"><param name="movie" value="http://www.youtube.com/v/vt2v3tpySSk&hl=en_US&fs=1"></param><param name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/vt2v3tpySSk&hl=en_US&fs=1" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="640" height="385"></embed></object>[/YT]
<object width="640" height="385"><param name="movie" value="http://www.youtube.com/v/vt2v3tpySSk&hl=en_US&fs=1"></param><param name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/vt2v3tpySSk&hl=en_US&fs=1" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="640" height="385"></embed></object>
Paste it on the reply, and then highlight the whole code and click the youtube button on the top tool bar.
[YT]<object width="640" height="385"><param name="movie" value="http://www.youtube.com/v/vt2v3tpySSk&hl=en_US&fs=1"></param><param name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/vt2v3tpySSk&hl=en_US&fs=1" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="640" height="385"></embed></object>[/YT]