I have shot the two sorcerers, read the inscription on the wall in the ancient language, crouched before the dagon statue (is this supposed to be the same as kneeling?), and pressed the six buttons in the proper order. But the door does not open. Is there some step I missed?

well there is not much to do after you kill the 2 sorcerers, so you have to press the buttons, but since the door is not opening, my guess is that you are pressing the wrong buttons. (if i can recall you are suppose to press the buttons on the broken slab, but they have to be decyphered by the codes in the dagon statue)