Hold [Left Shift], then press [F3] [F4] [F3] [F3] [F4] at the main menu. Then, press [F1] to display the cheat menu.
Cheat Mode (Alternate):
Start the game as usual. Press ` (tilde) to bring up the console. Type sam_iEnableCheats=1 and press [Enter]. Close the console (press ` again) and then bring up the main menu by pressing [ESC]. Access the cheat menu by pressing [F1].