This post is using UBBCode alone:
People appear to be having a problem with this simple task.
Here is a Pictorial.
First, in full reply mode, find the Markup drop-down dialogue box --

Then, select 'using HTML and UBBCode'

After you have that done, it's a simple matter of writing or acquiring the Embed Code from the source webpage:

below is the embed code, wrapped in quote markers so that you can see it:
<object width="425" height="344"><param name="movie" value="http://www.youtube.com/v/DFLgFwkYPVA&hl=en&fs=1"></param><param name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/DFLgFwkYPVA&hl=en&fs=1" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="425" height="344"></embed></object>
below is the code simply pasted into the posting, without the HTML markup option selected in the markup dropdown:
<object width="425" height="344"><param name="movie" value="http://www.youtube.com/v/DFLgFwkYPVA&hl=en&fs=1"></param><param name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/DFLgFwkYPVA&hl=en&fs=1" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="425" height="344"></embed></object>