Hi Karoline,
Have you received any official information from your university about enrolment yet? If you haven't, you could email your sup saying you haven't heard anything yet re enrolling and wondered whether there were any introductory events scheduled for new PhD students, and if not, could you arrange to meet up with him briefly to plan ahead for the term. I wouldn't worry about not hearing from him since July though, the summer is probably the slowest period of the year in universities, with staff starting to return about now for the new academic year. Also, if you have contact details for any of the postgrad research department (other than your sup) you could always give them a ring and ask about enrolment, it should be their responsibility to help you start, sort out any admin and get you settled, to some extent.
They're not daft questions either, btw!