Open the file called config.cfg located in ~/Library/Application Support/Euro Truck Simulator 2/. The full path is ~/Library/Application Support/Euro Truck Simulator 2/config.cfg. You can open this file with TextEdit. Once the file is open, look for: uset r_fullscreen. If the value next to it is 0, change it to 1.
It should read: uset r_fullscreen "1"