You can beign this sidequest as soon as you complete The New Kid in Town and can start exploring South Park. Mr. Slave's house is the red one and is located just south of the Tweek Bros. coffee shop in the upper right corner. He wants you to grab a package for him at the Post Office.
When you get to the Post Office, go behind the counter and you'll see the item he wants hopping around in the bin on the left side. Grab it and return it to Mr. Slave to finish the quest. Your reward is an item that can be used to summon Mr. Slave in regular battles (not in boss battles), and it can only be used once per day (you have to get another from Mr. Slave if used).
