when I open the console and try to use ANY input (from graphics to cheats, everything) it always says "unknown command".
Can someone explain me exactly what do I have to write and how, please?

?

console is appearantly locked down.

go to your CRYSIS2 installation root folder (the folder called \Electronic Arts\Crysis 2)
and make a file called "autoexec.cfg"

in that file put your commands that you want to use, seperate by each line.

EG:

r_HDRLevel = 0
cl_FOV = 80

..etc