the window should pop up. pick the propery then enter the value. whatever you click on after will have that prop/value "painted" onto it.
That's how PROP is supposed to work in the beta. It gives you a window, where you tell the game what you want to cange, and what to, then you just use it like a brush over all the pixels you want to change. Much better than the old way IMO.
Also, unable to reproduce @Pigbear 's problem on Mac ß.