Normally, enabling cheat codes will disable the ability to unlock achievements. To circumvent this, activate the desired cheat codes, then quit to the main menu. From here, choose "Continue" or "Replay Mission." Your cheats will still be in effect and achievements can be earned.
Somewhere in the middle of the first level of the game is a boarded up doorway that's not too hard to see off the main path. Claw through it and you'll find a hidden area with a powerup and what seems to be the hatch from Lost. Stand on top of it for a few seconds to unlock the Found! achievement.
In chapter 2, the 'What I Do Isn't Very Nice' checkpoint, there is a part where Agent Zero talks to you through a transmitter. At this part, you will cross a log over a gap, then climb up a small edge. Here, if you look to the right, there will be a log that leads up into a cave where the sword from World of Warcraft, Frostbourne resides. Wait there for a few seconds and the trophy will be unlocked.