It should be right next to the soap dispenser.
Do you mean the rinse aid dispenser? It should be right next to where you add the detergent
Some rinse aids are put in the door, and open with wash water. My detergent comes in single serving pods, it gets thrown in the bottom of the washer. Some rinse aids also go in the bottom. Look up the model number / maker on the dishwasher and see if a Manuel is on line.
Many dishwashers have caps to the rinse aid dispenser cavity that look like those in the first two pictures below. Depending on your appliance model, the cap, or lid, may look more like a plug, similar to the other pictures.
If you have the second type, you'll see threads inside the compartment, to indicate maximum fill line. Do not overfill this compartment, and wait until level is very low before refilling.