Enter the code on the third NOD mission, hold Z,L, and R and type the code
Perform the BARRACUDA cheat, then go to "Replay Mission" and press L. Now enter a mission and hold L + R to see the debugging mode.
At the title screen, press B, A, R-shoulder, R-shoulder, A, C-right, Up, Down, A. This work with both the analog stick and the + control pad. There won't be any confirmation, so just trust me. Anyway, after you do this(you must do it fast, before it goes into a demo), go into Replay Mission. Once there, press L-shoulder and voila! All the normal missions and som others.
At the title screen, press B, A, R, R, A, C-Right, Up, Down, A. Then, during a mission, press the following buttons: