It's a little bit difficult to explain how to see the pictures in 3D, but here are the instructions I put on the Flickr page:
To see these pictures in 3D, start to cross your eyes very slightly until the two images become three distinct, fuzzy images. (If you can count 4 distinct pictures, you're crossing your eyes too much)
Now, while maintaining those three images, try to relax and bring that middle image into focus. (This is the hardest part to learn.) When you get it right, your eyes will sort of "lock on" to the middle image and you'll see it in crystal clear 3-D.
Try not to sit too close to the screen. The closer you are to the picture, the more you have to strain to see it.
If you can't get it, at first, keep trying! It's hard to learn, but once you get it, it's very easy!