Type [Esc] to get to the main menu. In SYSTEM OPTION open the console,
 type in

 REMOTEADMIN 1

 and

 LOGIN developer

 This enables the cheat mode. Now you can type in (1 enables and
 0 disables the desired effect):

 FLY 0/1           Fly Mode
 GOD 0/1           God Mode
 NOCLIP 0/1        No Clipping
 UAMMO 0/1         Unlimited Ammo
 enable crosshair  Crosshair
 enable counter    Frame rate