how can I make it play with a certain deck?

I see no deck selections on my hearthbuddy. After I click start button, it doesn't give me a chance to choose the deck I want to play with. The result is that it always plays with my only existing deck, while I meant to level my warrior with its original deck.

BTW, what I bought is a 3 day trailer edition, which only offers me one session feature. What's the meaning session here? what's the difference between a one-session and a three-session deal?

how can I make it play with a certain deck?