Setting up video calling for Facebook requires you to download an add-on which will be available when you click the video icon during a conversation. For the button to be available, you and your interlocutor need to have web cameras connected to the computer.
The procedure is the following: click the video icon and wait for the website to load while it asks you if you want to enable video calling. If you accept it, an add-on will be offered for downloading. Download and install it and the page will automatically load. Then the Video Calling function will notify the other user to do the same and when this completes, you will see each other with video and sound.