Using Linux or OSX you cannot use the Configfile without an work around:

To work around: just create a file in "$HOME/.local/share/Colossal Order/Cities_Skylines/Addons/Mods" which is called 'TerraformTool\TerraformTool.xml' instead of the file TerraformTool.xml in the folder TerraformTool.

Remember, if you use the Commandline to create this file, you need to use: "TerraformTool\\TerraformTool.xml" or 'TerraformTool\TerraformTool.xml' (the difference is the usage of " or ')

The Bug is located in the ConfigData.cs.

this line:
string text = Path.Combine(DataLocation.modsPath, "TerraformTool\\TerraformTool.xml");
should be (compared to other mods which seem to work with Windows and Unix):
string text = Path.Combine(DataLocation.modsPath, "TerraformTool/TerraformTool.xml");