Ryan,
An excerpt form the .pdf installation/wiring file posted on the cnc4pc.website ...
PAGE 1 "Input and output pins with close by ground connections.
Forget about grounding problems. Easily connect your pin using your close by ground
connection..."
PAGE 3
"Testing:
Test the operation of the card and its connection to your computer and software before
connecting more hardware. To test output signals connect a LED and resistor between the
output pin been tested and ground, then activate the signal from your software. To test input
signals connect a wire between the pin been tested and ground, then read the input on your
software."
This means 0V should be applied to this input. Maybe you have a bad board?