I hate these topics. If you just played the damn game there's no way you wouldn't unlock it.
Beat Peace Walker 3. Complete all required ZEKE parts, add railgun. Play 3 (I think... something like that) Extra Ops missions and Zadornov will escape. After each Zadornov capture, play up to five EXOPs missions to have him escape again. He escapes 6 times. The final escape mission, he's in EXOP 01. Meet him and start the final battle.