not sure on the amount of Muriatic--but I would start with a pint (you have a big pool), wait a couple of hours, with the pump running full steam, then test again, then add some more. If you've had a lot of movement on the pH, but aren't "there" yet, next add a cup. If you haven't moved much, go with another pint. The object is not to overshoot your target.

DE: Diatomaceous Earth (probably spelled it wrong), the active component in DE filters. It's a very fine white powder only a little coarser than powdered sugar. It's the skeletons of little, long dead critters. DE filters have a reputation for being the finest (as in smallest particles) filtering medium.