Open the folder where your Marlin files are located and check if Configuration.h is present.
If it is it might be that when you load the Marlin.ino in Arduino the file is hidden. You have to click on the top right arrow and browse the list.
If you can’t find it just download the firmware again.[/quote]
it should be located between marlin.h and cofiguration_adv.h in the tab/file list.