What should I do about AG2R Mondiale?

Shortly after registering as an auto-entrepreneur in August 2011 I received a letter from AG2R Mondiale in which it mentions

Dans ce cadre, nous vous accompagnons pour repondre a vos obligations en matiere de protection sociale dans le domaine suivant”.....the letter continues with Sachez qu’il est obligatoire de vous affilier, meme si vous n’avez pas de personnel;  aucune cotisation ne vous sera reclamee tant que vous n’aurez psas de salerie.

I am not sure exactly what the letter is about but know from your previous blogs that other auto-entrepreneurs have also received similar letters. The letter is not requesting further information from me or wanting my RIB number etc. They have attached a form headed Bulletin D’immatriculation Au Regime De Prevoyance, they also mention in the attachment Contribution Obligatgoire Des Entreprises Relevant De La Convention Collective Nationale Des Professions De La Photographie Du 31 Mars 2000. They show my NAF code as 7420Z and my activitie economique as Activites Photographiques.

A few weeks later in September 2011 I registered a change to my situation stating that my main job was selling toys with my photographs in second place. My APE code was changed to 4789Z autres commerces de detaile sur eventaires et marches. What should I do, if anything?