I am away from my computer now, but the advanced mode reveals node parameters that aren’t visible in the simple mode (you can still select the nodes and press ctrl+h to reveal them, even if you didn’t enable the advanced mode). Some of those settings for the dirt node are AO, top to bottom, bottom to top, sides, top, and side texture stretching.
You can find the add-on files by default on c:/users/your-username/appdata/roaming/blender foundation/blender/2.xx/scripts/addons/grungit/grungit_data/Grunge maps/ (assuming you use Windows, on linux it’s usually ~/.config/blender/2.xx/scripts/addons/grungit/grungit_data/Grunge maps/).
Just remplace those maps with the maps of your choice with the same name.
This is not supported currently, you have to manually edit the dirt node group to fit your needs