First off, download the cheat enabler from
http://www.actiontrip.com/cheats/farcry.phtml and unpack the DevMode.lua file to the main FarCry directory overwritting other file (make a backup of it!).
Now you need to modify your desktop shortcut. Right click on the Far Cry icon on your desktop, select properties and a new screen should pop up. Add the " -DEVMODE" parameter (without the quotes) at the end of the target field so it looks similar to this:
"C:\Program Files\UBISOFT\Crytek\Far Cry\Bin32\FarCry.exe" -DEVMODE
Click Apply to save the changes and run the game using that shortcut. All levels will be unlocked and you'll be able to use the following buttons:
[F1] - Toggles first/third-person camera modes
[F2] - Move to next checkpoint
[F3] - Spawn point
[F4] - Toggle no-clip mode
[F5] - Default speed
[F9] - Save current position
[F10] - Load current position
[F11] - Toggle extra information
[=] - Increase speed
[-] - Decrease speed
[BackSpace] - Toggle God mode (if it doesn't work for you use the God mode cheat from below)
o - Gives 999 ammo
p - Gives all weapons
On that page you can find even more cheats
But play the game without cheats than yo'll enjoy it.