<< I'm stuck at the level where if you get detected, the mission is scrubbed.. I'm knifing everything in sight, sneaking around, and I still get noticed.. :| >>
There are 2 ways to go about this level. Cheating and No cheating.
Without cheating you have to walk all the way around the first part (hit N for the journal for hints)
The other way is to cheat and use /noclip
Fly around, shoot all the alarm boxes and everyone who appears except that last guy in the base. Best way to not get spotted is to fly under the terrain.
Also be sure to pick up your sniper rifle outside the base. If you don't you don't finish the round.
Fly back to near the start, kill everyone in the first section, walk through to the door inbetween the 2 sections, go up it, walk around the back, snipe the 2 guys under the netting and the one in the tower.
Walk a little further, snipe the guy on the water tower. When you walk up to the gate watch out for the guy in the guard house and the second guard on the loading platform.
Sneak behind the generators slowly, crouch and walk onto the truck.
It's not precise and takes a few tries but it worked for me both ways.