The animation may take a moment to load. During that time you may see nothing, then a Java "coffee cup" image, some various rectangles on the screen, and then a progress bar ...
Once the Scratch project has completely loaded you'll see a single cat.
This animation starts automatically,
with two messages from the middle cat
Click the green flag to restart the animation
To stop the animation click the red "stop sign".
Click the cat to make her hop. Change the number of hops with the slider near the top (this changes the value in the orange field.)