What you actually need to do is:
ctrl +shift + c
type: testingcheats on
(in the cheat window it will say "cheats are enabled")
then type: cas.fulleditmode
(nothing will come up in the cheat window in response to you typing this)
DO NOT close the cheat window yet (It won't work if you close it now)
with cheat window still open, shift + left click on sim, select edit in cas or modify in cas (I forget which it is) - it's on the right of the options.
Then when you're in cas you can close the cheat window.
Make your changes, click the tick.
When you're back in-wold, open the cheat window again, type: testingcheats off
(in the cheat window it will say "cheats are disabled")
close cheat window.
edited for more info.