The Mendeley Web importer allows you to automatically add references from websites, databases, and catalogs to Mendeley. To install the Mendeley Web importer, simply click the red "Get Web Importer" button, which will redirect you to your browser's extension database. From there, click whatever "get extension" appears on your browser.
To use the Mendeley importer, click the Mendeley icon on your browser's toolbar at the top right of the screen when you're on the page of a database catalog.
In Chrome and Firefox, browser extensions can be found in the puzzle piece icon in the upper right corner of your window. The Zotero connector icon should be there, and you can click the little pushpin icon to "pin" it to your taskbar. In Safari, the extensions menu can be found under the "Safari" menu in the bar at the top of your screen.
Chrome extensions menu:
Firefix extensions menu: