There is currently no option to allow users to select categories. We'll keep the feedback in mind for future updates.

But it was mentioned on the concrete5 site...

There are categories in the backend that an administrator can assign to testimonials. What I mean is that there's not an option for for end users to select a category when submitting a testimonial. So if you are using categories, normally you would assign a category when approving the submitted testimonial in the backend.

And when I test te site in a html validator I get two errors. I'll enclose the results.

The error for the label is the result of changing the ID of the field element to avoid CSS conflicts. It shouldn't affect much, but I will fix it in the next update. The other error is not a part of the block's html, it looks like it's probably part of your theme's html.