First you need to place tiles that produce terrain yields. This is basically rivers, forests and most importantly plains. Then you use the environment tool to turn 3x3 tiles into kingdom or fallen environment. After this the tiles should be settlable. Place some chasm terrain somewhere to make the map maker overlay update the new tile yield values.

Since tile yields are dynamic you cannot directly alter them in the editor. You need to either make a new terrain type or create a world resource with the purpose of giving extra tile yields for this.