Rules for self-scheduling (when implemented): you can only have one "slot" at any given time. You can't select an occupied slot unless you first persuade the occupant to move to an unoccupied one. Two talks on the same Experiment must be scheduled on different days to prevent "upstaging".
While that feature is not yet working, the Peer Marking part is! Once you have logged in, go to that form and click on the name of the person you're ready to mark; a marking form will pop up to make it easy for you to record your opinions. (We keep records of who has marked what, so don't mark anyone twice!) You should also mark yourself.