I've beat Veteran level.
You already have the card key from the desk in the 1st bldg on the right (just past the tent),
and you already have the explosive pack from the last (3rd) bldg on the left.
Enter the ammo dump bldg. which is the last building on the right. (Straight across from the bldg where you got your explosives). I never encountered the main door into the bldg as locked.
Inside the bldg there's a locked door to the cage w/ ammo. I assume you refer to this door in your OP. You don't really even need to open that locked door unless you want to go into the cage & replenish your stuff. Your card key opens it (F key).
Look at the pile of boxes OUTSIDE the locked cage, but still inside the same big room as the locked ammo cage. You should see a faint red spot ON THE PILE OF BOXES where you plant your explosive (with the F key) then run.
Then go to the single bldg way off in the distance and take either of the 2 2-door buggy jeeps. As you drive past that bldg it's doors open & you shoot 2 attackers who drive out behind you.
Then you must leave your 2-door buggy and instead get into their 4-door jeep, because it has a rocket launcher (X key) you'll need.
P.S. I looked at your rig specs in MyAnandtech. How come you're running that nice AMD 3500+ and that good OCZ RAM at only 2200 MHz? Those were made to overclock, man! Why not 2600MHz or more?