Play multiplayer

I found the answer in this thread, especially this post helped a lot. It's unpleasant for me to answer my own question, but there came no one to help.

You need to enable the Developer Mode in order to be able to use these commands.

To enable the developer mode, you need to start the game with the "-devmode" command line parameter (either edit the start menu shortcut or create a batch file containing "Crysis2.exe -devmode". When the developer mode is enabled, you'll be able to use the following key bindings:

• F1: toggles between first and third person view
• F2: teleports you to the next checkpoint
• F3: switches between fly mode, noclip mode, and back to normal
• F10: removes many textures and shows polygons
• Num1: gives quite a few weapons (notably the JAW and L-TAG) and refills ammunition

Update: After some research it has been found out that the "-devmode" cannot be used after patching the game to version 1.2. However, there might be ways to bypass this through using scripts and/or similar.

Source: Wiki4Games