Go to the Home screen and mouse over the blue quest. You have to complete Sparky’s Challenge (3 wins vs AI) to progress to the Starter Deck Duel event queue that will unlock the 10 dual-color starter decks. This is a dual-color precon queue against other players that only uses the dual-color starter decks. It’s good for new players because you can even complete your dailies here.
Alternatively, if you’re experienced in MTG and want to skip all this, just hit ESC (Options) -> Account (bottom left link) -> Unlock Play Modes
Getting the 10 dual-color starter decks has changed recently and you now get free golden packs after buying 10 of the latest main set (e.g. buying ‘March of the Machine’ with gold. ‘Aftermath’ also contributes to golden packs but is a supplemental set so not as good)…