Go to "Setup-bp" in your main S.T.A.L.K.E.R. directory.
Choose your language and for Password # 1, input "pseudodog" to unlock the multiplayer map Pool as well as the UK and US military multiplayer skins.
For Password # 2 input "snork" to unlock the multiplayer map Dark Valley as well as the French and German military multiplayer skins.
These two codes are exclusive to the Limited Edition of S.T.A.L.K.E.R.