Are you asking about calibre or calibre companion? My guess is calibre, and you are looking at Préférences / Avancé / Extensions, then possibly "Obtenir de nouvelles extensions".
Under the assumption I am right I will move your question to the calibre forum. I can move it back if I am wrong.
As for your problem: check that your firewall permits calibre to use the network.
Moderator Notice
Moved to the calibre "Plugins" forum