You need to use the shift key and the LMB when lowering the terrain.
The change log has all the changes and how to work the stuff in it. it is in the root folder of GameGuru here:
C:\Program Files x86\The Game Creators\SteamLibrary\steamapps\common\Game Guru
Intel(R) Core(TM)2 Quad CPU Q6600 @ 2.40GHz, 2400 Mhz, 4 Core(s), 4 Logical Processor(s), 8gb RAM, Nvidia gtx660, Windows 7 Pro 64bit