This function returns the name (as a string) of the currently selected configuration for your game. For more information on configurations please see the section Advanced Use: Configurations.
if os_get_config() = "Free_Version"
ads_enable((room_width / 2) - 100, 0, 200, 75, 60);
The above code will check to see which configuration is being used and if it is the one called "Free_Version", ads will be enabled in the game.