To access your cards, login to your account and tap "Post Box" at the bottom of the screen. Upcoming events will be featured at the top of your postbox; you can scroll to the right to see these cards.
You can scroll down to see your other folders. You can click on Drafts, Sent, or Received to open up each folder and view your cards.
To access all of your cards, tap the account menu at the top left of the screen, then tap "Post Box." You'll then be able to tap between your upcoming events, photo galleries, sent and received cards, and online drafts.