In order to enable the flycam in American Truck Simulator, you will need to locate your config.cfg file. This would normally be found in this directory -> /Documents/American Truck Simulator/
Once the config file is located, you can open it with the text editor of your preference. You can then search for uset g_developer "0" & uset g_console "0"
By default these will both be set to value 0, but you will need to change both of their values to 1 to enable the flycam. After all of this is done, you can save the file and launch your game.
Hope this helps.