I cant seem to get into any doors in warehouse A, ive tried punching their numbers off the map into that machine but it doesnt unlock em and no key ive found unlocks em either

i really suck getting stuck on the 2nd mission too..

Are you trying to open more than one door at the same time? Only one of the doors can be open at a time, so if you put one code in, then another without going to the door, the first one will close again.

there's an elevator in one of the buildings. shoot the button with an arrow and it will come down.

also, there's a way behind the big crate boxes

from the top of roof/balcony area:
also you can break glass windows with sword, and break all non-metal doors with the sword if you don't have a key/cant pick it

just hold the attack button until Garrett grunts and pulls the sword back then hit the door, do this 3-4 times until it breaks open.
Of course this approach makes a lot of noise and can alert guards.