If you look at carefully to the documentation, it states explicitly that you'd need to load your data:
...
data = load('vehicleDatasetGroundTruth.mat');
vehicleDataset = data.vehicleDataset;
That means what you should do is to create data store of your images: see DOC how to create data store images.
Step 1. Create a data store from your images using: imageDatastore()
Step 2. Load your created data store (Optional: to create one .mat file and then load the file is optional. Only if you want to keep/store all your created data sets in one *.mat file)
Step 3. Start simulation
