If you have the same ones I own, they automatically grind the salt/pepper when inverted? It was sold as a pair and has the black top, clear lucite-type container for the salt or pepper, then the base where the battery compartment is in the bottom? (Mine is stainless, but they sold copper colored too). The black top twists, you have to look at where the arrow lines up. There are two arrows about 1/2 inch apart. On mine, you twist the top to the left and that releases the top so you can open it and refill the container. Don't fill it above the line or you'll have problems putting the top back on and having the grinder work properly. Then re-align the arrow and twist back to the right to the next arrow to secure it.
If you have a different model, I'm sure they all work in a similar fashion. Mine did come with an instruction manual, if you still have yours, I'm sure it would explain how to fill them.