On this page of the game guide to Borderlands 2 you will find a detailed description of the side mission titled The Overlooked: Medicine Man. You can start this quest in the Sanctuary location during the completion of the main mission named Bright Lights, Flying City.
The Highlands - northern part
2) Karima's house
4) Vending machine
The Highlands - southern part
6) Container with Shivers Medicine
Accept the mission from Scooter  and use Fast Travel to get to The Highlands Overlook. There, approach the house in which Karima lives . She will ask you to activate the Vending machine. For that, you will have to head to the top of the clock tower  using the ladder shown in the above screenshot.
From the roof, head to the container (screenshot) where you should find the Power Supply. With it in hand, start the Vending machine  and buy Shivers Medicine. Now leave the area and head to the north-east part of the map .
Here you will meet the merchant from the above screenshot, together with Robot guards. Eliminate them all and use the Vending machine on the merchant's back to buy another Shivers Medicine.
The last medicine can be found in a container by the bridge in the southern part of The Highlands . In order to get inside, use the button on the right side of the door. The whole area is protected by Threshers who you can either ignore or kill. Having gathered all the Medicine, return to Overlook  and deliver the medicine to the marked houses. Speak to Karima to end the mission.