Put the meter into DC Volts mode.
There should be two probes from your meter.
One probe should be plugged into the + (pos) port on the meter. Put that probe in the middle of the plug.
The other plug should be plugged into - (neg or ground) port on the meter. Touch that probe to the outside of the plug.
Your meter should read somewhere between + 11.85 to 12.15 volts. If it reads - (negative) or outside the range, then no go.