To find it, look for a fairly large, black rectangular box with numerous connectors. It's often bolted to the inner fender well or a nearby support structure. You might need a flashlight and possibly even to move some wiring harnesses to get a clear view.
Consult your 1993 Dodge Dakota's repair manual. This is the most reliable way to pinpoint the ECM's location for your specific vehicle. A Chilton's or Haynes manual would be a good place to start. Online repair manuals are also available, but make sure it's specifically for a 1993 model year.