Look for a heavy-duty cord, usually black, that protrudes from a fitting on the engine block. It will have a plug on the end designed to connect to an external power source (usually a standard 110V household outlet).
If you can't find it readily, check your owner's manual for a diagram showing its location. You could also search online for specific images of a 2007 Dodge Cummins engine bay to help visually locate it.