Can someone explain how to use these properly and what the levels should be in DVRack/OnLocation? There is no black, just gray tones so I'm not sure what I should be trying for to achieve proper exposure.

Thanks

The goal is to have the black chip of the card as low as possible on the waveform, and the white chip near 100%. That way you are exposing the maximum latitude on the camera. It should show a so called 'Stair case' on the waveform monitor.
One must say that DSCcharts are much better then the carton printed stair case which is provided with DV rack. But it does it's job too. Make sure to aviod reflexions on the card when you are shooting it.