My first guess is you do not have your deck equipped, young Wizard. Just like a hat, you must have the deck equipped in order to use it.
To equip the deck, press B on your keyboard when you're not in a duel. This will bring up your Backpack. Click the tab at the top that looks like a deck. Make sure the deck you want to use has a small closed hand icon beside it, indicating that is the deck you currently have equipped.
Once the deck is equipped, make sure you have cards in it! This and lots of other helpful information can be found in the Players Guide at http://www.wizard101.com/guide
