How do you get to Ames in Metal Gear Solid 2 sons of liberty?

You keep on the disguise you have to wear in order to fool guards, take out the directional microphone, and listen to the heart beats of all the hostages. If you're listening and you hear a beeping noise, that is Ames. You also have to find an AK-74U in strut F's warehouse or else you're not gonna be able to enter shell 1 core's elevator.