After some puzzling (don't be too politie, just open the door and walk into the courtyard) there was a envelop on the door of our appartment with instructions. They were also send by BNB-mail and by SMS but we missed these because of a netwerk failure. The appartment is very nice, very good for a family of 4 and on a terriffic location; in the middle of Trastevere but very quiet at night.