Live subdivision is applied to mesh objects. To enable it, click the 'Node view settings' button (it has an icon that looks like an eye), which is located in the top left of the 'Mesh' tab in the toolbox (see previous tutorials in order to create a mesh). This will bring up the Node view settings popup. There you will see the live subdivision settings.
The 'enable' check turns live subdivision on and off. The 'iterations' entry controls the number of subdivision levels. The 'max faces' entry limits the subdivision such that it will not subdivide beyond this number of faces; this prevents the program from freezing due to subdividing too far if the number of iterations was set too high by accident. The 'normal sharpness' radio buttons controls how normal sharpness is preserved. The 'wireframe mode' controls which subdivided edges are displayed.