After completing the "You've Gotta Shoot Them In The Head" mission return to Mr. Crowley once you have all four keys. Follow him out of underworld, and take back the keys. Go to Fort Constantine and enter the little house on the left named the "CO Quarters". Go downstairs, and use the key on the "launch control bunker" door. You will find doors for all the keys. The T-51b is in the last room under glass. Use the computer to free it.