By default, the Control Point toolbar provides tools for modifying, adding, and deleting control points for applicable markups (such as the Polyline, Polygon, Arc, and Cloud markups) or measurements (such as the Area, Perimeter, and Volume measurement markups). These tools will be unavailable/dimmed if the selected markup cannot have its control points modified. See Working with Control Points for more information.
If the toolbar is currently hidden, go to View > 
Toolbars and select it to show it.
Add Control Point: Adds a new point where the mouse is clicked.
Subtract Control Point: When clicked on an existing control point, deletes the point while leaving the shape.
Convert Control Point: Click to toggle between a corner and an arc; click and drag to control the arc more precisely.