How to load a struct file after labeling positive images using training image labeler for training a cascade calssifier?

hello im new to matlab, I'm trying to train a cascade classifier based on the Train a Cascade Object Detector. I've collected around 100 positive images using the training image labeler , that gave me a '.mat' file. But when i want to train the classifier using this:

trainCascadeObjectDetector('stopSignDetector.xml',positiveInstances, ... negativeFolder,'FalseAlarmRate',0.1,'NumCascadeStages',5);

I get an error I'm trying in the 'positiveInstances'. It says that an error occurred, not sure what argument i should pass and where to get it, all what was mentioned is that it should be a struct type from the positive samples. Any ideas where i can get it?

Many thanks