It is great to see the cast screen option in the drop down setting shade in 5.1 but it can never find either of the Chromecasts.
Am I doing something wrong or how do you use that option?
My phone is on KK and it has the Cast screen box in the drop down menu, it works in conjunction with the Chromecast app, do you mean that 5.1 added screen mirroring independent of Chromecast?
It does have screen mirroring natively but what I meant was, you will need to set up the Chromecast devices through the Chromecast app. In case you have already done so disregard, there's another issue preventing your streaming.