Install the latest version of the AIR SDK as described here: Managing the AIR SDK

Download and unzip Starling (to an accessible location on your system)

Launch Animate CC.

In Animate CC, go to “File” → “ActionScript Settings…” → click the “Source path” tab on the “Advanced ActionScript 3.0 Settings” dialog.

Click the button “Add a New Path”.

Enter the “starling\src” path to the location where you unzipped Starling or browse to the location by clicking the button

Click Ok.

You now can use Starling with Animate CC!