You can open the window by going to Window > Rogo Digital > Get Extensions.
You can click a category in the yellow bar at the top to filter the extensions in the list. You can also change the order extensions show up in by using the Sort by toggles on the left.
Each entry in the list shows the name of the extension, the products (LipSync Pro, LipSync Lite and Eye Controller) that it works with, a description and the version number & last update date. When you've found the extension you want, click the Download button to its right. A progress bar will appear at the bottom of the window. When it completes, Unity will begin importing the downloaded assets - you'll have to confirm the import.
A list of available extensions can be found in the Extensions category of this documentation.