if its the level where you have to find and escort the vip then heres how you do it, when you turn a corner and see the plaza (or whatever it is) the building that friendly snipers are in, its the building accross the street from it and a little to the left, you just need to walk up to the building and you will see on the bottom of the screen the L1 option to plant C4, you just need to press L1 and get away from the explosive.