When you download a ATS mod file it will usually come in one of three file types: .ZIP , .RAR and .SCS

Unzip the .ZIO or .RAR archives into your “My Documents\American Truck Simulator\mod” folder. If you have .SCS, just copy file to the same dir.

Don’t forget to activate mods in your game.

Question: How can I install a Mod (zip file) in Mac OS X?
Answer: Copy the zip file into the following folder (don’t unpack the ZIP file): /Library/Application