My very brief reading of the highlighted part of the manual suggests to me that you can control the indexer without needing any input from the keypad - just send step and direction signals to it with an Arduino. And you could use your Visual C++ program to make whatever sort of user interface your want and have it send a message to the Arduino which tells the Arduino how many steps to get the indexer to move.
That seems to me a lot easier than trying to intercept key-presses on the keypad.
As far as I can see the indexer is just a stepper motor driving a screw (probably a very carefully manufactured screw 
).
...R
Stepper Motor Basics
Simple Stepper Code
Serial Input Basics - simple reliable ways to receive data from your PC.