All decks are empty. You must place the spells from your pack into the deck yourself, and don't forget the Treasure cards!
To put a card inside your deck, simply click on it on the right side of the screen and it will show up in your deck contents. The red circles next to the card shows the max number of copies you can put inside the current deck. The yellow circles show the number of that card that you have already put into your deck.
If you want the prize, you have to build the ladder rung by rung. There are no shortcuts.
Also, remember that Treasure cards stay with whatever deck you put them in. If you switch decks, you may want to remove the treasure cards from the old deck and then put them into the deck you're using.