I have the same problem, I am logged in, and dragged a rectangle around the area I wanted to download , and the button was still greyed out , did you ever get a resolution to this?
Could you please provide a screen shot and step by step instructions on how you got to the screen shot so that we can try to reproduce it? Did you start with a new scene? Please also include the version of CityEngine that you are using.
I got the same problem yesterday.
My project was using a Local Coordinate System (UTM Portugal).
Solution: Create a new projec without Coordinate System or use Geographic Coordinate Systems and the button will work again.
Having this issue now. ALso not defining it but it sets it to a default set. Did you end up setting it to Preferences>secene>Scene coordinate system>choose>No Projection > Raw data in feet/meters ?