Mouse Modes: Slicer has three main mouse modes that apply to the 3D viewer: Pick (which allows "pickable" objects to be selected), Place (which permits objects to be interactively placed and positioned), and Transform (rotate, pan and zoom). The toolbar icons that switch between these mouse modes are shown from left to right above, respectively.

Hint: These modes are persistent when selected; when you are done "picking" or "placing", make sure to select the Transform mode again.