If the key was shut off and all you got was half a volt testing between the neg terminal of the battery and the frame, then you don't appear to have a defect such as a short in the system unless your battery only has .43 volts capacity. Typically seeing a very low voltage is because a clock is running or something similar that requires very low current and runs all the time.
Before testing with an ammeter, you need to place the probes in the proper ports for amps on your meter. Select your highest amp setting (as Ed mentioned) on the meter and attach the leads between the negative terminal of the battery and the frame with key off. If you get nothing, lower the amp setting until you do. If you don't get anything, you don't have a draw. The next step would be to see if you have a defective battery. Typically fully charging the battery then letting it sit for a week would indicate if it was good.
