Click this button on the top toolbar to toggle the pop-up window.
The controls will be disabled until some 2D shapes are selected. Click the 
add button to show a 3D representation of the selected 2D shapes.
Once the 2D shapes are added, they are deselected and the add button disabled.
If you select a different set of 2D shapes and add them to the 3D view they will replace any shapes already present in the 3D view.