[CutSickAss]

I'm not sure I understood you correctly, but if you mean you're seeing something like this:




then, as it says on the top, you press RIGHT, and when you see the play button you press SELECT (Enter) for the trailer:




If the trailer plays in the background, just press the TAB button for it to come to the foreground (you can change this option in Settings/Skin).
Also, if you press LEFT it brings up the Media Menu; unchecking the Trailer Button will stop showing that big "Play" sign on the poster.