Dear Herbert Ströbele,
Thank you for your mail. Since you are a speaker for this session, you should be able to add materials already for this session. If you click on the pencil icon on the right of "Presentation Materials" a menu should open that you can use to upload files.
If you have any problems with this, please let me know. You need to be logged in on Indico for the icon to appear.
I have added a page about the discussion session to the website (https://indico.cern.ch/event/576735/page/11135-discussion-session). This
page also links to the event in the timetable.
Best,
Lennart van Doremalen