The set of pins used to control the shield are pretty straight forward and you should be able find them described in the documentation for your shield. You need to find the piece of code that coverts the abstract idea of 'turn motor on' to a set of signals written to output pins. This will presumably be in your VB or Firmata code. The documentation will tell you what signals you need to give the shield to make it do what you want. The VB/Firmata code will show you what it's actually doing. You need to see what the difference is, and fix the code so that it does the right thing.
Code samples showing how to control the motors directly from a sketch running on the Arduino may help you to confirm you understand what pins do what, but the code you need to fix is NOT Arduino code; it is your VB/Firmata code.