There are 3 different tabs at the top of this webpage.
This is the page you are on now. Here you can verify that the Plex server is working. If you click "Launch Plex" it will start Plex in your web browser.
Click "Request Something" to bring up the requests page. Just type in your Plex username and click "Sign In." From here you can search for Movies and TV Shows to have them added to the Plex server.
Click "Whats New" to bring up the recently added page. Here you can view what's new on the Plex server this week. It's a web version of the weekly email that you receive.