Here's the thread for Magic Duels (which I think is the game you're talking about):
http://www.neogaf.com/forum/showthread.php?t=1076927
The game's very grind-dependent if you want to unlock cards outside of your starting pool. You get six cards per pack, and they cost 150 in-game coins, which equals to winning ten duels against hard AI with its random decks (which isn't all that hard and awards you 15 coins per win) or eight wins against human opponents (20 coins per win).
There is a deck wizard that forces you to take whichever cards it wants, but you can also build your own deck from scratch. The only big restriction in Duels is that the number of copies of a card you can have in any given deck (or open in booster packs, thankfully) is directly proportional to how rare the card is (commons: 4; uncommons: 3; rares: 2; mythic rares: 1 per deck).
While it isn't all that fun after a while, I'd say it's somewhat decent for learning the game or getting back into it after a while without having to pay money upfront like in MTGO.