Hello Dan,
Thanks for a good question. If you want to use only your eyes to activate, you should look at the GazeAware behavior instead. The Activatable behavior requires a second input, such as a button, to control when the activation should be triggered.
You can look at the GazeAware3DScene to see how a script (GrowOnGaze, inherited from EyeXGameObjectInteractorBase) is attached to a 3D game object. I guess it should be equivalent for a 2D scene, but maybe we have missed something. Please let us know in that case.