If you are at work, here is the more subtle version (or back to the proper version).
Select a competition day at the top of the page. If you haven't used this simulator before, choose the practice day (click in the little circle next to where it says 'Practice' at the top of this page) and you'll see further instructions here.
There is a help file which gives more information on running the simulator, plus a detailed description of how the simulator actually works.