Why do I never see the Air Play icon in iTunes?

Why can I never use Air Play to play music from my computer on my Apple TV? I've had two different versions of Apple TV, the most updated versions of iTunes and have gone through all the trouble shooting suggestions and NOTHING has worked. What is wrong with Air Play and why has it never worked for me? The only thing it has worked on is my iPad Mini, but I don't keep music on there, so that doesn't really help.