You can only edit your deck when you're playing as Sora. The reason is that Riku's decks are always preset and can't be changed.
It's pretty simple, just add any card(s) to any of Sora's decks, then remove the card(s) 500 times. The catch is that you just can't add cards and remove them, that doesn't count. You have to go to a deck and add or remove any card(s), leave the deck editing menu (press the B button twice), select Review Decks, and add or remove any card(s) 499 more times to nab this achievement.