During gameplay press tilde (~)  to go to console,  then enter the following cheats: 

GOD 1 - God Mode 
UAMMO 1 - Unlimited Ammo 
GHOST 1 - Invisible 
FLY 1 - You Can fly 
NOCLIP 1 - Walk Thru Walls 
NOIMPACT 1 - ? 

Go into your game /settings directory. Find the levels.sav file.
(If you don't find, simple create a levels.sav file with some kind of text editor.)
Just put the following lines:

Mission1=yes
Mission2=yes
Mission3=yes
Mission4=yes
Mission5=yes
Mission6=yes
Mission7=yes
Mission8=yes
Mission9=yes
Mission10=yes

Save your levels.sav file.
Run game as usual and you will have access to all missions.