OpenATC is best viewed with Google Earth.
Click here to download the Google Pack containing Google Earth:Viewing Traffic in Google Earth
First, make sure you have Google Earth installed on your computer. Click here to download Google Earth for free if you don't already have it!
Then, simply 
click here and Google Earth will launch automatically and display your Air Traffic!
Useful add-ons:
-
Worldwide weather overlay (pretty low res): click here
-
Europe Lightning: click here