Part of the campaign, first plane (Innistrad) that you dual on, 4th match. If you're not sure how to get there, click on campaign, Innistrad, advance past the little text story that pops up and defeat the Moorland Zombies, Skirsdag Cultists, and Angry Mob.
You can also pick this one up along the way by losing to the Angry Mob deck:
If you're having difficulties with this one you can try to lower the difficulty to Mage in the Help and Options menu.