[flash=425,350]http://www.youtube.com/v/G4h7NGMz2RI[/flash]

Tutorial: how to embed YouTube videos.

Here's the code: *flash=425,350]http://www.youtube.com/v/3nrE_QF0mJo[/flash*

instead of "*" put "[" and "]" respectively to make it work.

http://www.youtube.com/v/3nrE_QF0mJo would be your video's URL, you can find it within the embedded URL that looks like this
"<object style="height:390px;width:640px">param name="movie"value="http://www.youtube.com/v/D9gYhnUKehU?version=3&feature=player_embedded"><param name="allowFullScreen"value="true"><param name="allowScriptAccess"value="always"><embed src="http://www.youtube.com/v/D9gYhnUKehU?version=3&feature=player_embedded" type="application/x-shockwave-flash" allowfullscreen="true"allowScriptAccess="always" width="640" height="360"></object>"

The green highlighted text is what you want.