They are absurdly simple machines, so much so that a manual almost isn't necessary.
This gist is this: Set the dial to zero, then look through the sight. Now, turn the silver knob sticking up out of the base back and forth (might go around a few times) to adjust the lamp height so that the center dot visible through the sight is the same brightness as the outer ring. It might help to squint a little.
To take a reading, put a negative under the sight and look through, then turn the dial until the inner and outer segments match again. Lean back and take note of the number on the dial, which is your density reading.