You will need to acquire three vials found in locked cabinets around the map. You will simply interact with them to open them, and pick up the item inside.
One is in a storage room near spawn, to the right of Quick Revive and to the right of the staircase.
One is in the Colonel's Office above Juggernog.
One is in Engineering. After entering the area to the left of the Helipad, go to the lower level of Engineering, near a generator to find the cabinet.
After acquiring all three vials, interact with a mixing device on a table in the Field Hospital, against the wall facing spawn. After interacting with the device, many Hellhounds will spawn during the mixing process. After completion, pick up the truth serum from the table.
You then need to locate an air conditioner outside of Military Command. Head outside and look directly at the OPC, and jump off of the railing to the left to find the air conditioner in a corner. Interact with it to start pumping truth serum into Dr. Peck's room. Then interact with Peck, and a long conversation will begin and end with him starting to dance.
After he has been dancing for a while, interact with him again and he will tell you to head to the Data Center.