I've attached a copy of the instructions for the probe.
I haven't used the probe for a while because I bought an Analyser Pro 500 (which replaces both the probe and control unit - highly recommended).
From memory, you measure an area of dark shadow (about Zone 1). You then make a print in which the measured area has the correct density, and then follow the instructions to calibrate the probe.

If you use several papers, you can use the Calibration knob to switch between papers - for example, Ilford may be setting 5, and Kodak setting 7.

Regarding the "HELP" message, the number indicates the fault:
2 Calibration knob between two numbers.
3 (or 5) Too much light to measure - stop down lens.
4 Not enough light to measure - open up lens.

To cancel the help message, press the Expose/Cancel button on the control unit.