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 -> Exposure" then click the lightning bolt above the camera view

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 annoying. 

Also it seems like the bluescreens can corrupt my maps since I get a completely empty map when I load the map I tried to build. This has only happened twice though while I've gotten plenty of bluescreens.

Specs:
Windows 10
GTX 980
i7 6700k
16GB DDR4 RAM

Running the mod tools on an HDD.