First use the silenced rifle to take out the guard in the upper gate house before proceeding to the compound. He really requires a head shot to kill him in one try, if not, squeeze off a second shot FAST.
Approach the compound from the far left side, entering through the side window. Use the silenced Sten to clear the room with the one guard in it.
Leave that building from the door nearest the dead guard, and head for the building to your right. Either enter it, or move around the side of it. There's a guard on top of the water tower, kill him using the silenced rifle.
While facing in the direction of the water tower, there's a gate behind you, you need to cautiously enter that gate. A soldier might open it and walk towards you, so kill him too w/ the silenced Sten.
The last guard before getting in to the truck that you have to kill is the one walking on the loading dock the truck is parked in front of. Using either the silenced rifle(if you still have ammo for it, or the scoped rifle, kill him. Run up to the platform from the far right and get on the damn truck!!
Good luck.