The counting clam Scratch 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 fish and a clam.
Click the green flag to play the animation (or to replay it.)
To stop the animation click the red "stop sign".
Typically Scratch animations are hosted on MIT's website. The source XHTML of this page demonstrates how to host a scratch.sb file on your own site or server. I've heard there are problems when hosting an .sb animation in HTML5 pages, but have not encountered them. Hosting works fine in HTML, XHTML or HTML5 pages.