At the main menu, hold down all four shoulder buttons (L1+L2+R1+R2) at once, for about 3 to 4 seconds, until the "Security Clearance" cheat menu pops up.
To enter the letters for the codes, hold down the X button over the character you wish to change, then move the + button left or right to change the characters, then let go of the X button to move to the next character using + button. Once all the characters are entered in correctly, press the Square button, and the cheat code should turn orange so as indicate that the code was entered in correctly. Press Triangle to exit to the Main Menu, then select "Previously On 24", Then Select "Missions", Then Load your Save File, Then select the "Missions" tab, then follow the instructions on the bottom of the screen.