You do not really need any additives if you use a good quality name brand diesel fuel. If you want there is one that some forum members use, it's called " Seafoam ".You can buy it at NAPA . Just add it to about half a tank and then drive for awhile and then refuel. You just need to do this once a year.This is to clean up your injectors and not your " carb "
Also if your diesel is running rough there might be other causes and the additive may not solve your roughness.