To enable these, you'll need a jailbroken PS3 (Custom Firmware) or a Devkit and v1.11 of the game (with region BCES01584/BCES01585). The offsets to unlock it through a PS3API or by making your own EBOOT.BIN are 0x00A92640 38000001980B2DB0, though a premade EBOOT.BIN can be found here (It's the one I made). In either case, place the file in dev_hdd0/game/BCES01584/USRDIR/ and start the game. Now, during gameplay press L3 + Start for the Dev Menu, L3 + Select for the Quick Menu, or L3 + R3 for Debug Fly. Some features aren't working, however, such as Player 1 and Player 2 options. You can enable them with hex modding however, but that's another tutorial for another time.

Dev Menu

Dev Menu can be opened with L3 + START. Here is its content:

Dev Menu -> Levels

allows you to set the difficulty at any given point in time.

Dev Menu -> Levels

I have no idea what this does to be honest.

Dev Menu -> Memory

There is too many things to explain here, just look at the screenshot, the options are pretty self-explanatory.

Dev Menu -> Clock

allows you to change the game's speed or framerate.

Dev Menu -> Tasks

allows you to choose any task you want to play in both the single-player campaign and DLC chapters.

Quick Menu

Has some additional options, such as Infinite Ammo, Invincible Player, and No Reload.