I'm assuming it's a youtube vid
1 Click on the youtube button.
Should look like this.
[youtube][/youtube]
2 in your chosen youtube video address bar, select the text after the "equal" sign and before the "&" sign.
http://www.youtube.com/watch?v=zAUXPkx7gMI&feature=related
3 insert between the middle brackets of the youtube text.
(youtube]insert text here[/youtube)
Should look something like this, (left a space on both sides, to illustrate)
(youtube]
zAUXPkx7gMI [/youtube)
Don't leave any spaces and it should be good to go.