First, you need to use the Insert Video button: Enter the s-less YouTube address there. Then highlight that, click , enter text ("Watch Trailer" or something), and clock OK, then post.
The template I have used for these is:
[SPOIL="Watch trailer:"][video=youtube;YOUTUBECODE]http://www.youtube.com/watch?v=YOUTUBECODE[/video][/SPOIL]
So, to clarify, the instructions at the top mention to use this specific spoiler tag, because it reduces page load with videos, but you need to insert the video first, and the pop-up box that comes up with the spoiler is asking for a spoiler description, thus "Watch Trailer". I hope this makes sense!