Sorry for bumping a kind of old thread, but did you manage to solve the issue? I just got this today, had no issues previously.

and i have seen that it says "sun not built" in the game view window. so im just wondering how i can build a sun

If you want to build the sun press "View -> View Toolbar -> Exposu...

Have you tried pressing F8 after loading the map?

You delete items with backspace (above enter). I haven't added zombie spawns myself yet so that I don't know.

So I've started messing around with Radiant a bit but I've been getting bluescreens when building maps. Even when the map only consist of a few brushes with only the dev texture I still bluescreen at times. It's not very consistent but it happens from time to time so it's pretty ...