Looking at the pictures I think this is how you're supposed to use it.
1. Make a good test print.
2. Select the range switch that is close to your exposure time of the test print.
3. Put the probe under the enlarger at the point you want to measure (it could be the shadow, highlight or midtone. it's your choice)
4. Set the sensitivity knob until the meter read the same time as your exposure time for the good test print.
5. Put an unknown negative on the enlarger and put the probe at the point where you want to be the same density as the good test print.
6 Read the time on the meter. This is the exposure time for your new negative. I presume if your new negative is much lighter or darker the test negative you could use the range switch to switch it to different range for better reading.

That's how I think it should work. I think it's a primitive meter and doesn't take into account of the dynamic range of the negative. It simply tries to reproduce the same density on the print as that of the test print.