I am not part of the Adafruit team but I think I may be able to solve your problem.
I use several of these meters (and the V-meter) on my solar-power set-up.
First, keep in mind that this is a current-meter. It is used to measure Amperes.
Or, on this specific meter - it can display a min of .00-A to a max of 9.99-A
A Volt-meter would connect from the + supply to ground but, not an Amp-meter.
To properly connect the 'Panel Current Meter' (ID-574):
You correctly supplied power for the meter from a separate 9-v battery.
Nothing wrong doing that.
However, you seem to have mis-connected the current-leads.
It is necessary to break a connection in the power supply lead and insert the meter.
One way to do this is to cut the (-) lead from the AC-adapter (or battery pack).
The meter's black-wire connects to the cut-wire that attaches to AC-adapter's (-).
The meter's red-wire goes to the other end of the cut wire that goes to the Arduino.
It is also possible to power the meter from the Arduino's AC-adapter (or battery pack).
To do this, connect the thin black-wire to the thick black-wire.
splice into the AC-adapter's (+) lead.
Now the AC-adapter provides power to the current meter as well as the Arduino.
If you are powering the Arduino from USB it is a little more involved to get to the
center power pins on the USB cable. But idea is the same:
Cut the USB cable and expose the USB power leads.
Insert the meter into the (-) power-lead.
Good Luck, John