I have 3dsmax 2018 my system. I've just ran the executable that I've sent in the first place.
I thought it would appear in the Tools menu, but since it is not there we have to run the script manually.
Just click on the Scripting menu and click Run Script.
The file selection dialog directly opened the corresponding Max version's Scripts folder and there is a Populate-Terrain sub-folder right there.
Click on the Populate-terrain.mse file in the sub-folder.
The Populate terrain dialog will appear. I once used it with terrain curves but I don't remember the details. I've tried with a few splines. Just selected and pressed Process. It said Invalid Boolean.
I think the ands of your severals curves must be connected with splines, too.
If you plan to use it so often, you can use the Customize User Interface command in Customize menu and add the Populate command to either the toolbar or the menus. You'll look for the Populate command in Main UI commands.
By the way if this terrain is a real world terrain, you can use depth maps of that area as a displacement map.
Tangram Heightmapper is a website where you can extract heightmaps.
