Each browser has its own version of the Zotero connector, so install the one for the browser you use.
Each browser puts the connector in a slightly different place. See below:
With Chrome, make sure you click the puzzle piece and "pin" the Zotero connector to your browser interface.
** Since the release of Safari 13, the Safari connector has been broken, due to Apple locking down their browser to extensions. If you are a Mac user, please use either the Chrome or Firefox browser and extension instead.