you should have some cards called "blank cards" among your battle cards by now.
these have a black name area, and a few dots on a black background as image.
if you try to add them to your folder you'll be asked if you want to activate it.
if you do, you enter a screen with a space background.
if you tap on the right places in the right order the card will become a battle card.
you can find out the codes for each battle card by either buying the wave commander kit thingy.
or by finding a website with all codes.