When in the corkboard view if you try to add more index cards by either pressing the green add button at the top of the screen, or either of the add text icons on the bottom left hand side of the corkboard window or binder window, you are able to add one index card. If you click to add a second index card you automatically drill down into the text of this second card. To get back to the corkboard view you then have to click on one of the textfiles listed in the binder and then on a folder listed in the binder which takes you back to the corkboard for that folder.
The only way to add multiple index cards in the corkboard view without having to constantly switch back from text view in this way is to add an index card and then move the cursor to the corkboard itself and click on the corkboard before adding another index card.