When you visit Facebook you should now see a panel appear in the bottom right, with a button to switch back to the Old Layout.
You should now see the Old Layout icon in your browser toolbar. Click it to get to information about the extension. If you don't see the icon, it might be collapsed. Click the puzzle piece to see all your extensions and "pin" it so it shows up.
The extension stopped working due to Facebook's changes, but we're updating the extension with new functionality soon.
Keep the extension installed to benefit from future features!