Try the parallel view. I can't cross my eyes at will either. I can try to force it by focusing on something (a finger for example) between my eyes and the screen but as soon as the object I'm focusing on goes away, I focus on the screen again. Parallel view is looking at the screen but focusing on the distance. If you can see any stereogram of this kind, you can see this.
Oh btw, PAUSE THE VIDEO WHILE FOCUSING and then continue the playback. Much easier.
I take it you're not using the colored glasses view.
Google for "stereogram" and try to go through some tutorial. If you can see stereograms you can see this, either in cross-eye or parallel view.
Edit: Wollan, try zooming out on the webpage, ctrl+minus on firefox and ie I guess. It's easier for me with smaller pics.