Well, great news. I fixed up some code on the remote controller part which I built, and then on the robot itself. I hooked up a 9V (1.5V x 6) battery pack to the ext pwr jack on the motor shield, with the jumper on. Any it works! Not sure what was going on when I was using hte power suplly within the robot's chassis, but when I used an independent one external to it, it works.
I should say that I originally tried it with a 9V battery also plugged into the Leonardo's barrel jack while the (1.5V x 6) battery pack was plugged into the ext pwr, and saw no performance difference at all. In fact, I then removed the jumper, and again, no difference in performance. So, that's when I tried only leaving the (1.5V x 6) battery pack plugged in, with the barrel jack on, and it works!
Thanks everyone for your help! Had to just keep working at it, and it works!