the air play icon does not apear in ITunes. How do I get air play to work?

I have been trying to get Airplay to work on my computer. When I go to support on Apple it mentions clicking on the Airplay Icon but the only problem is that the Icon is nowhere to be found. I am running itunes version 11.1.5.5 on a windows 7 operating system on the computer. Airplay works just fine if I am streaming directly from my Iphone or Ipad to my Sony Stereo, or Apple TV. My Apple TV is running version 6.1 and it can see my computer when I select it on the menu. How can I stream my music from my computer using Itunes to my Apple TV?