Once downloaded and pinned, extensions appear on the top right of your browser, next to the search bar. Use them by clicking the icons:
Use the puzzle piece to choose which extensions are "pinned" (show on your browser). You can also add delete extensions here:
Pins that are visible on your browser are solid blue. Pins that are installed but not visible on your browser are grey and white.
You might unpin an extension if you only use it occasionally to keep your browser from getting cluttered.