The DLC consists of three missions. Only the first mission (Slaughterhouse) and the second mission (Regal district) have favors to buy. Once you start the mission you will be met with a menu asking if you want to buy any items, upgrades or favors. You need to buy all 3 favors on the first mission, and both favors for the second.
Note; you don't necessarily have to use the favors. IE Some will have NPC's dropping off runes for you about the map, You don't need to find the rune, you just need to purchase the favor.
You start the DLC off with 2000 Gold. This is way more than enough to buy all the favors, provided you don't go spending crazy on the first mission.