You don't need to download anything, assuming that you have Google Earth installed. On the OpenATC page, click the "Show Airtraffic" box at the top of the screen. Then click "Google Earth". Then click where it says: "Then, simply click here and Google Earth will launch automatically and display your Air Traffic"..
When Google Earth appears, ensure that you tick the selection box on the left. You should see lots of triangles with flight numbers next to them. They are aircraft and their positions are 5+ minutes behind.