For some Welcome Week events you need to book your place on your Timetable and through a separate booking form. Often this is because you need to purchase a ticket or sign an additional form. You'll be alerted to this at the top of your Timetable, it'll look like this:
Please click on the 'Book place' button and you will be taken to the relevant webpage. You can then click on 'I have booked' and the alert will disappear.
Make sure you do this soon as your place is not booked until you have follow the 'book place' link and follow those instructions.