In order to edit the custom directives go to RhoToops > Custom directives

Group: Select the Build Target Group
Player Settings...: Opens the Player Settings menu
Create directive: Creates a new directive
Synchronize: Set the "Scripting Define Symbols" as the ones with ticks on the list.

Under "Synchronize" you can see the current configuration.

The toggle in the list will determine if the directive is added to the configuration
Copy will copy the directive to clipboard
The "x" button will delete the directive

All directives are saved in the directory "CustomSettings" in the project root.