Select an Object in a 3D Scene
You must select an object before performing many different actions.
Get in interaction mode 
.
Click the object.
A white contour line indicates that the object is selected.
Note
The patient model is the exception in this case. No contour line is shown when the patient model is selected.
In the left image no objects are selected. The image at the right shows a selected wax-up with a white contour.