Make sure that you are in the right level -> as far as I have seen the map was made with the world composition tool. So choose the level with the landscape and then try it again
When the map uses the world composition tool, the world is split into different levels-> so this means that it could be that you are currently in a level which just contains a special part of the landscape. So you cant paint onto it with the foilage tool, as just one part is active. You will have to select the level with the landscape and then you should be abel to paint onto it with the foilage tool.
I have them all loaded and viewing all in one piece, even selecting it with the landscape tool i cant paint with foliage. here is an image showing that: ://i.imgur/80dfBPl.png