It’s likely that your Zette is fully downloaded, but nested in your Extensions Menu and not pinned to your browser toolbar. In order to easily see the Zette extension at any time, you’ll need to pin it to your browser toolbar.
Click the jigsaw puzzle piece icon on the top right corner of your browser to open up your Extensions Menu, and simply click the pin icon next to Zette. The Zette icon should then pop up right in your browser.
See below for a reference photo.