Use a text editor to open "armyops.ini" file in the system folder of the game 
directory. Find the phrase [Engine.GameEngine] and add this line: 

 bMPCheats=Enabled 2 

Now you can press the TILDE key while playing to display the developer console.

Result                          Code 
---------------------------------------------
Third person view             - behindview 1
Unlimited ammunition          - mpcheat paramsammo 1
No clipping mode              - mpcheat ghost
Disable no clipping mode      - walk
Weapon select                 - mpcheat changeclass [weapon name]
Start game alone1             - start [level name]
Spawn indicated item or NPC1  - summon [item or character name]
Freeze game play              - playersonly
Remove all characters         - killpawns
Make some spawned NPCs walk   - walk
Single player map select      - open [map name]