The Hammer and The Shredder were pre-order bonus exclusives that were never made available post-launch. However, you can finally unlock them by placing the two DLC files in your "resources" folder (located where you installed F.E.A.R 3). Both The Hammer and The Shredder will be available under Game Settings. They're only usable in multiplayer and replace your last stand/"down but not out" weapon. The Hammer is (according to the wiki) Paxton's revolver, and The Shredder is a non-lethal double-barreled shotgun.