If there is already a valid card for that event in the archive, the system will show it to you, allowing you to select it via the USE button (the green botton):
If, on the other hand, you do not yet have any cards stored in your personal profile (menu under Avatar, DOCUMENTS) or those that are present are not suitable (due to discipline or expiry), the system will allow you to re-register and subsequently archive them by clicking on the 'ADD' button
NB. The system will only propose memberships that have a valid deadline for the event and whose discipline is relevant to it!