Adding lights in Hammer Source L4D.
1. Make sure you have a light source. A lamp, sky or fire are examples of light source.
Using the adding props tutorial, add a dynamic, static or physics light source.
2. Entity --> Light, Light Directional, Light Spot, Light Dynamic or Light_Environment
Light: basic light source
Light_Spot: creates a spot light effect, directed light
Light_Directional: a dynamic light_environment
Light_Dynamic: light that can be moved and changed
Light_Env: is used as a sun or moon, you must have a skybox for this to take effect.
3. Select the light entity and Alt+Enter for Light Properties Option.
Here you can change the color, intensity and many other options you see listed.
For more detailed explanation for each one visit wiki.
4. In 3d viewport under camera, you are able to view 3D Lighting Preview and 3D Ray-Traced Preview.
3D Lighting Preview: doesn't reflect exact light you will see in game, but it is a good start.
3D Ray-Traced Preview: renders a screenshot of your view and provides a better accurate lighting in game.
This will only work if you have 3D Lighting Preview turned on.