Here's how I do it: you'll notice that there's a repeating pattern, cross your eyes until you get two of the repititions to be on top of each other. Once you've done this you should find that the shapes pop out.
To properly view them, you need to go wall-eyed, by focusing on something behind the screen. This is something that I have not been able to do yet.
It's a good technique if the repeating strips are really wide. On my screen at least, the strips here are pretty narrow, so it should be relatively easy to converge your eyes correctly.
Having said that, I don't know if this script is resolution independent, so it's possible that on some monitors the strips are a lot wider than they should be and that could be giving people more trouble.