Go to Contribution List in the left hand menu and then click on the title of your presentation.
Click on the pencil icon next to Files and then Add Material.
Please note that you need an account (a cern external lightweight account or a cern nice account) to upload your presentation.
If you are not already in possession of one; please register for an external lightweight account through this link;
Once this is done return to your e-mail in order to confirm this request !
If you already have an account, but there seems to be a problem: please refer to the same link and/or contact the CERN helpdesk (+41 22 767 8888).