Hi,
the probe is an input into Mach4.

The UC100 in a sense mimics a parallel port and replicates a parallel port numbering.

pin 1   output
pins 2-9 outputs
pins10-13,15 inputs
pins 14,16,17 outputs.

You need to decide which of the very few inputs you have to spare, pins 10-13 and 15, and connect one of them to your probe.
Then in the UC100 plugin you need to connect that pin to Machs Probe signal.

Craig