Bring 20 Werewolf Meats to Gruun and speak to him about Business. He will take them in exchange for the Key of Sevens 5. If you have completed the Golden Circle quest line, he will give the key up without needing any werewolf meat
If you have a high enough renown (somewhere between 25-50) he will say he has heard about you and give the key for free.
Gruun has several quest buttons that do nothing. Most notably, "Wanted" which is what the Werewolf Meat description says to press. Pressing Business completes the quest instead.