Hello Guus,
1) Make sure you are connected as an admin
2) Go to "manage the calendar" in the admin panel
3) Press and hold your mouse on a time slot (it's quite similar to Google calendar) >
The following dialog box sould be dispalyed
I've just tested it on the demo site and it works. Tell us if it works for you and feel free to open a topic to introduce yourself.
regards,
Pascal