Once you go through the grate behind the guard, grab the wire clippers and go back through the grate. (You can't use the security panel without a card, so don't bother with it.) Use the wire clippers on the barrels to create a distraction - just be sure to scoot to the left after you do it, 'cause the guard will run over to the barrels and you don't want to get caught.
While he's preoccupied with the barrels, go back to where you started (STAY CLOSE TO THE DOCK!), shimmy up the pole, grab the backpack, and get back down quickly. Open the backpack in your inventory; you'll find something very useful...