Q: I don't have HomeKit on my iPad Air

HomeKit is not an app, but a developer framework for controlling connected devices.

https://developer.apple.com/homekit/

Have what? Did you read my response And the link I posted?

See if this article makes sense to you: Use HomeKit-enabled accessories with your iPhone, iPad, and iPod touch - Apple Support

In short, if you have iOS 8.1 or later, you can use HomeKit enabled device/accessory + an app for that device (you'll get this from the app store).

Maybe he ment something different.

We have 4 devices ). 2x Pad air  and 2x Iphone5S  all with iOS9.1  

on the IOS device  ->  SETTINGS   just below SAFARI  i see the Homekit-Icon.

Strange though, only on one  iphone (5S iOS9.1)  there is NO Homekit-Icon ?????