To add a card, enter the information for your card in the text field under one of the available sections (To Do, Doing, Done) and then hit Enter on your keyboard.
Click on a card to make changes. A popover window will appear for editing the selected card.
You may edit the Card Title and Description from this window and also make other changes such as assigning people, adding a checklist, and so on.
A card may also be deleted by using Delete (or Fn + backspace for Mac OS) on your keyboard.