Ello, go to the garagegames website and download the free trial
(
TGEA Demo)
When starting the demolevel, press F11 to start the World editor. You can play around with the loaded level or go to File -> New Mission to start a new Level with Terrain.
Press F4 to place objects, F3 to edit them and F11 to switch to switch between edit mode and play mode.
EDIT:
Broozar, the Forge Demo is the new demo. It looks like a small watchable demo only, but just press F11 inside and then start a new Mission.