Sounds like your deck is not equipped, young Wizard. Just like a hat, you have to equip a deck to be able to use it. Press B on your keyboard to open your backpack. Click the Deck tab. Double click your Deck to equip it. Open your Deck and make sure you have cards in it, and don't forget the Treasure Cards.
Once you have a deck equipped, you need to place your spell cards within the deck itself.
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.
