The experiment file is set up to make 600 1-second counts and plot a histogram showing frequency vs count interval.  You can superimpose a poisson curve in Graphical Analysis to assess the agreement.

Use "counting histogram" experiment file.

Have the counter set to the appropriate operating voltage for the geiger tube.

Use a sufficiently active source - the calibration source from the old survey meter works well.

Make sure the axes and bins are scaled appropriately.