Instructions
Find the game's 3 MO disks. Since these are essential story items, you will find these disks naturally as you play. Once you get to the "Umbrella Labs" portion of the mansion, you should have three disks.
Go to the morgue in the third-level basement of the labs and place a disk in the MO terminal you find there. Go to the fuel storage lab in the third-level basement of the labs and place a disk in the MO terminal you find there. Go to power room #2 in the third-level basement of the labs and place a disk in the MO terminal you find there. For each MO disk you use, you will receive a passcode.
Go to the main hall of the third-level basement after fighting the game's final boss and try to open the door with three locks. Since you have all three passcodes, the door should open automatically. After you do this, a scene will occur in which Jill frees Chris from the jail.