I had the same question a couple of weeks ago. Tempest explained it well here. You should find that "sweet spot" on the side of the coffin and push down on the joystick. You will hear a sound that indicates you checked it. If a cross is inside it will appear in your inventory box. There is (I think) one cross on each of the two "coffin" levels. So once you find one you can move on.
I had trouble killing Dracula at first but with the kind help of these veteran gamers here I was able to do it easily. First position yourself in the upper right corner. Wait until Drac approaches and swing below him. Now this was the part I was getting wrong. I was pressing the button expecting it to repel Drac for awhile. You must hold down the button and it will repel him in the opposite direction. In this case, it will push him up, since you are below him, and into the prison in the upper right-hand corner. If you run out of the effect of the cross. Press and hold the button again because you should have three crosses at the beginning of this screen. One for each coffin level and one for entering Drac's room.
Once you do it you'll find the hardest screen is probably the 'shooting gallery' screen which you have already past.