Successfully complete the "You've Gotta Shoot Them In The Head" mission given by 
Mr. Crowley in Underworld. Return to him once you have all four keys. Follow him 
out of Underworld, and take back the keys. Go to "Fort Constantine", located in 
the upper-left corner of the map directly between and slightly to the north of 
"SATCOM Array NW-05a" and "SATCOM Array NN-03d". Enter the little house on the 
left named the "CO Quarters". Go downstairs, and use the key on the "launch 
control bunker" door. Be prepared to fend off robots while making your way 
further down. Eventually you will find doors for all the keys, the last one 
being a room that holds the T-51b under glass. Use the computer to free the 
armor.