To use codes, hold down CTRL and the following number:

0 - Finish mission
1 - Invincibility
2 - Unlimited torpedoes
3 - Unlimited canon ammo

Cheat Edit
[ Warning: If you are unfamiliar with editing via Notepad and you edit incorrectly the game WILL crash. So be careful and back up the saved game you are editing ]

To edit, open a "Save Game" file in Notepad. To get to a saved game file in Notepad this is the string: Bluebyte\AD\Dat\Save. Then open the saved games until you find the one you want. For example: if you want to edit a saved game you named "Floodgate" open that saved game and the first 3 lines will read:

[Global]
Name = Floodgate
Credit = 13237 <- this line shows how many credits you currently have

You can change this line i.e.
Credit = 99999 <- you will now have $99,999 when you load this saved game

This is enough money to buy anything you want from any Arms Dealer.

Further down in the saved game are the the codes for the ships, weapons, etc.
You can edit just about everything including the missions.

[Another Warning]
Editing anything other than the money (usually) involves changing more than one line. This is where it gets tricky and where you face the greater possibility of screwing up and crashing the game. Or, find yourself in a "Dead End" where you can't go anywhere.