Interactive placement is done using the light properties dialog box.
To open the light properties dialog box:
Click the Start button to interactively position the light around your objects. The light will maintain its distance to the current point of interest, as if sliding on the surface of a sphere centered on that point. Moving the point of interest will change the radius of that sphere.
When you have moved the light to the desired location, click the same button in the Light Properties dialog, now labeled Stop.