First unlock the game's console by going to Game Options and select "Yes" for the Game Console. Next open it up with the ~ key. Then, type in these codes to go to the level.
------------------------------------------------------

Devmap pby_fly --> Play the mission Black Cats

Devmap see2 --> Play the mission Blood and Iron

Devmap oki2 --> Play the mission Blowtorch & Corkscrew

Devmap oki3 --> Play the mission Breaking Point

Devmap pel1a --> Play the mission Burn 'Em Out

Devmap ber3b --> Play the mission Downfall

Devmap ber2 --> Play the mission Eviction

Devmap pel2 --> Play the mission Hard Landing

Devmap ber3a --> Play the mission Heart of the Reich

Devmap pel1 --> Play the mission Little Resistance

Devmap nazi_zombie_prototype --> Play the mission Nacht der Untoten

Devmap pel1b --> Play the mission Relentless

Devmap ber1 --> Play the mission Ring Of Steel

Devmap mak --> Play the mission Semper Fi

Devmap see1 --> Play the mission Their Land, Their Blood

Devmap sniper --> Play the mission Vendetta