When you want to embed a video into the forums. 

1.  Switch to HTML mode.  If you use the Rich Text Editor, it's the tab next to the pencil at the bottom of the editor.  If you use the Basic Text Box, it's the HTML mode button.

2.  Place your embed code.

3.  Remove the last tag.  (If there is an Object or Param tags, remove them as well.  See the video for an example.)

That should work.

Peace
K

One last item from me, YouTube's default size breaks the forums because it is wider than the forum is.  If you've been here for any period of time, you know what I mean.  When you embed the code, there's a spot in the code that sets the size, change the width (which will probably be 480) to something like 350 or so.  It may look a little funky when you embed it (unless you also change the height) but people still have the option to run it fullscreen, so it doesn't really matter.  That will keep the thread from breaking so people can actually respond to the video.