Be sure the simulation has finished loading before you begin. Please note that I know this does not work...
http://www.compadre.org/Physlets/mechanics/
For a prelab, you may be given a link to a physlet and there will usually be a question that you then need to answer about the simulation. You may start and stop the simulations using the buttons on the bottom of the animation. You can interact with the simulations in various ways: click and drag inside the animation to read the coordinates. Click on the link above for an introduction to physlets: Under "Chapter 1: Introduction to Physlets, click on any of the links and make sure that you understand how a physlet runs (and you might check to see if it runs on your computer).