To make plugin works you have to at least (settings page):
– turn on “Automatic watermarking” for uploading new images or “Manual watermarking” for images already in Media Library,
– set watermark image
– set image sizes on which watermark should appear.

Did you refresh page with your images after watermarking? Sometimes WP shows old (cached) images. Try to force-refresh (CTRL+F5) site or download images (sizes selected in settings) and check them for your own.

If you still have problems please respond.