Decks are unlocked by beating the owner in the campaign. Defeat all of the Planeswalkers in the main campaign to unlock all of the decks. Note that there are a handful of Planeswalkers that you have to defeat who aren't part of the proper campaign progression. Look at the Planeswalker encounters in the bottom right-hand corner of the screen. You'll find Jace Beleren, Ajani Goldmane, and Lillian Vess there. Beat them to get their decks as well. "Deck Keys" can also be used to unlock decks without having to play the campaign.