Unreal4.3.0 not found…
I think it’s still experimental. You will have to enable it in your project / editor preferences (not sure which one it was).
Under Editor Settings, in the General Section there’s a section named “Experimental”, just tick the box for Behavior Trees and you’re good to go.
Be warned that to my knowledge, since it’s not finished, there’s no substantial documentation on it, if any at all. That said I think there’s some tutorials on Youtube about it, but I haven’t been through them so I can’t be sure.