I need a dummeys guide to the editor

[xing]

I can't find where you paint creeper onto the map

[teknotiss]

that's because you can't yet, it's intended for a future update i believe, but V has things to do before that so don't hold your breath  ;)
so if you want C on a map you can place emitters, and AC emitters to keep areas clear, and then run the game for a bit and delete the AC in the editor terrain panel.
presto a map with C and a clear zone, but no AC.
hope that helps 8)

[J]

If you really want some creeper at the start of your map, use CRPL (you can even use CRPL to render the second frame so it shows up (unconfirmed))

[Grauniad]

Yes, use a CRPL core with a simple SetCreeper and then call destroy on the unit.