You need to select the object, and the surface on the object, that you want to emit light - for example the tube of a neon light, or the screen of a TV or monitor. You can do this in most cases using the Surface Selection tool (it may fail if, for example, the TV has a modelled glass cover to the modelled phosphor layer - in that case select the object and then select the surface from the list in the Editor tab of the Surfaces pane). Once you have the model and surface selected double-click the preset for the light you want to apply.