You are using shaders every tiem you render. If you mean Shader Presets, you need to select the item you wish to apply them to and the surface on the item, then double-click the preset; usually this is easiest to do in one click with the Surface Selection tool (the potential issues are where the surface you want is underneath another, as with the inner parts of the eye - for those you need to select the model and then get the surface selected via the list on the left of the Surfaces pane, in its Editor tab).

The Surfaces pane (Window>Panes(Tabs)>Surface) is where you manually adjust the settings for the applied shader (Editor tab) or you can also use it to apply presets (Presets tab)..