There is nothing to open and nothing to set up. Open any video and you will see a camera button in the player controls, next to the settings gear.
Open YouTubeAlready have a video open? Switch to that tab — the button is there too.
Where the button is
Open a video on YouTube, or switch back to the tab you already have open.
Look at the right end of the player controls, just before the settings gear.
Pause on the moment you want and click the camera. The frame lands in your downloads folder.
Keyboard shortcuts
AltS capture the current frame
Alt← step one frame back
Alt→ step one frame forward
Set the frame rate to match your video. A 60 fps clip needs the 60 setting, or the arrows land on every other frame.
The settings live behind the toolbar icon
Capturing a frame needs nothing from the toolbar, but everything else is in a panel that opens from the extension icon: the image format, the frame rate for the arrows, and whether a frame is saved or copied to the clipboard. Chrome hides that icon under the puzzle piece, so pin it once and it stays in view.