The memberships are a little tricky to find and are only available in the Playtomic mobile app, not the website version viewed from a computer.  
Download the app using the link below, scan the QR using your smart phone camera; click the link and download the app. Open the app & search for Harbour Padel and scroll across the menu that says
Home, Book, Open Matches, Competitions to find Memberships on the right side! 
Click on Memberships and the options of Gold, Junior, Platinum and Silver are listed.
Select the one that suits you best and you're all set.