If there is a Check Engine light on, then there is a Diagnostic Trouble Code, then it has to be fixed. The code will give a diagnostician a good road map towards the fix that is needed. It is unlikely a clear the code fixes the problem. That's why it's illegal to do so (for a pro).
To get the correct scan tool to view / clear codes, you will need to know what protocol your vehicle uses. You can call Cummins with your VIN and they can assist.
Neal is correct, typically it is at the base of the steering column. It is likely a round connector about 1 1/2" in diameter. It may (should) have a cap on top of it. There also will be one in the engine bay, typically near the rear bumper for easy connection.
If you are looking for an "OBDII" connector, not sure yours has that. But that is a parallelogram connector that GAS vehicles use since 1996. Maybe you can't find it because it was built with a Diesel Deutsch connector (the round 9 pin one) instead.