As this is a .scs file, you have to put it in C:\Users\username\Documents\American Truck Simulator\mod (If using Windows 10, just open the documents quick link and copy/paste the .scs file there). Start ATS and you will see it in the mod library as the game will detect the new file and tell you "New MODs found".

How do I get it to work do I just add it like a mod or do I rename it and put it with the dlc packs