In a large bowl, combine the masa harina and water with your hands to form a smooth, pliable dough. If too sticky to form, mix in a little more masa harina. If too dry and begins to crack when formed, add a little more water.
Divide the dough into 6 equal pieces. Form one piece into a flattened circle. Add about 2-3 tablespoons of the cheese to the center, leaving room around the edges. Form the edges together over the filling and gently roll in a ball to seal.
Lightly press the ball, working around the center to form into a disc. Push between the hands and fingers, back and forth, to form a circular, filled pupusa about 1/2 inch thick. Repeat with remaining dough and cheese.
Place a large skillet over medium heat. Once heated, lightly oil and add 1-2 pupusas, depending on the size of your pan. Cook until golden brown blisters form on the bottom. Flip and cook on the other side until the blisters form. Transfer to a plate and repeat with remaining pupusas.
Serve the pupusas hot with the prepared curtido and salsa roja.