When you start the game select the mission where it says Quaker Jack and his henchmen are stopping construction of the bridge. When you appear on the level, walk along as you normally would, until you get to the first hole in the ground.Step down onto the ledge and shoot at the wall on the other side of the hole. The word Go should appear with six flashing asterix's around it. Jump onto the word Go and the informer should pop up telling you about the secret mission.(Note that if you choose to do this secret mission,once you have completed it you will be put back to the start of the mission that you selected where Quaker Jack has taken over the bridge).