Add "sky" "###", "_sun_angle" "# #", "_sun_diffuse" "#", "_sun_ambient" "#" and "_sun_light" "#" to the worldspawn.
These dafault settings work well with the riverscape1 sky:
"sky" "riverscape1"
"_sun_angle" "200 -30"
"_sun_diffuse" "60"
"_sun_ambient" "30"
"_sun_light" "250"
Other sky names are:
unit1_ = Orange sunset sky (change angle to "_sun_angle" "72 -25")
pbsky1 = cloudy sky (change angle to something like "_sun_angle" "198 -65")
pbsky2 = sky with sun (change angle to something like "_sun_angle" "150 -34")
pbsky3 = starry sky (change angle to something like "_sun_angle" "85 -38")
pbsky4 = bright sunrise by hills (change angle to something like "_sun_angle" "90 -30")
arctic1 = snowy mountains and ocean (change angle to something like "_sun_angle" "35 -30")
sand = desert (change angle to something like "_sun_angle" "228 -40")
(or any other skies in your "\pball\env" folder)
To find the optimal sun angle for your map, you can also put your crosshair on the sun, then type "angles" in the console, and it will tell you what angles to use for the sun.
Make sure to run a final compile. Otherwise the sun lighting doesn't work (well)!
http://www.digitalpaint.org/testwiki1/pb2wiki.php?word=entities