just to be clear we are doing the same I will put a step-by-step protocol to analyse your image:
1) open the picture
2) convert to 8bit (Image->type 8bit)
3) set the pixel calibration properly from Image->Properties you will set 1 pixel is 1 pixel.
so now you will have a gray picture with the calibration in pixel on the top part of the picture you must have 598X897 pixels
4) set the threshold with Image->Adjust->Threshold and I keep the Default method, but maybe you can try someone else: don't click apply keep it with the red (you can set it in the threshold tool selecting red).
5) now you can analyse particles with Analyze-Analyze Particles...
usually I just add them to manager and if you want you can directly add to the Results Table (just pay attention when you repeat the analysis with another picture to delete previous rois and results to don't count them double times)
*the black balls will be 373 (the number of measurements/rois you) have divided by 3 rounded up -> 373/3 = 125 *
Hoping that is clearer and helpful
have a nice day