You can log in to your personal customer account here. There you can view details of your booking as well as add equipment to your booking.
Click in the navigation on the left on "Add equipment". There you will find an overview of the available equipment:
Select the desired equipment and click on "Book now" on the right side. The equipment will be added to your booking as soon as the payment is confirmed.
You can find a list of existing equipment here: What equipment does my camper offer?