For M.A. and PhD students, the student ID you will receive also works as your library card.
To receive a library card as a PostDoc or Professor, take your confirmation of your status as a visiting scholar at Freie Universitaet Berlin to the counter of the Philological Library in the Rost- und Silberlaube. They will give you an application form. It usually takes 1-2 hours to process your application. You can then collect your library card, which entitles you to borrow books from all libraries of Freie Universitaet Berlin.
Saturday - Sunday 10 am - 8 pm