You can turn the sun on by adding key/values to any "Worldspawn" brush--like a skybox brush. Highlight one and hit key N, then add the values below.
Key: suncolor
Value: 65 65 65
Key: ambientlight
Value: 17 17 17
That should give you fairly bright daylight, inside and outside. If you want the shadows darker, reduce the ambient light. Suncolor 70 70 70 is considered full sunlight.