I think the screenshots on the add-on page do a well-enough job of explaining how to use the extension. If you can't see the extension button near the URL bar you should probably pin it (Click on the "Extensions" button that looks like a puzzle piece that you can find near the settings button, right click this extension and then tick "Pin to Toolbar"). This is, as far I can understand, standard behavior for all Firefox extensions. If you think it's bad design then go take it up with them I guess :)
Feel free to send me an e-mail if you need further help with this