All Duke Nukem 3D cheat codes are to be used when you are playing the game. These are not command line, menu, or console commands.
Section I: Useful Cheats (these affect gameplay)
* DNCORNHOLIO - God Mode (And Unlimited Jetpack).
* DNKROZ - Same as Cornholio, just quicker to type.
* DNVIEW - Same as the F7 key in the game. (Also prints the message "You're too good to be cheating")
* DNHYPER - Puts you in Steroids Mode.
* DNITEMS - Gives all inventory items and keys.
* DNWEAPONS - Gives all weapons and ammo.
* DNKEYS - Gives all the keys.
* DNSTUFF - Gives all weapons, keys, inventory items.
* DNSCOTTYxyz - Level Warp. x is the Episode Number, yz is the level number. (Ex: dnscotty103 for Episode 1 Level 3, or dnscotty211 for Episode 2 Level 11)
* DNSKILLx - Changes the difficulty mode to x. (x can be 0 to 3)
* DNUNLOCK - Opens all doors and unlocks all locked doors.
* DNMONSTERS - Turns off (or on) the monsters in the game.
* DNCLIP - Walk Through Walls
Warning: Being in God Mode can crash the game at certain points, depending on what you are doing. Being in God mode when you are squashed by rotating gears is one example.
Warning 2: The DNCLIP code can be very dangerous. It can cause crashes at any time. If you have a problem, and have used DNCLIP, restart, and do not use DNCLIP, and see if it happens. This was the main reason we didn't have a DNCLIP in v1.0.
Section II: Cute Cheats (they do something, but don't affect the game)
* DNSHOWMAP - Shows the entire map.
* DNCASHMAN - Spews money everywhere when you press space.
* DNCOORDS - Shows extremely detailed map coordinate/level information.
* DNDEBUG - Shows some debug information.
* DNRATE - Shows your frame rate onscreen. You must be in full screen mode for this to be anywhere near accurate.
* DNBETA - Prints the message "Pirates Suck!"
* DNALLEN - Prints the message "Buy Major Stryker"
* DNTODD - Prints the message "Register Cosmo Today!"
Section III: Useless Cheats
* DNXXXXXX - Does Nothing.
Notes
* The DNCOSMO cheat was removed in v1.3d, because the keystroke conflicted with a game function. DNTODD in v1.3d is functionally the same as DNCOSMO.
* The DNUNLOCK cheat code does *NOT* work in shareware. It only works in the registered version of the game.
Notes
* "Customer Cheat" means it's the cheat given out with the registered version of the software.
* "Debug Mode" means it's an unsupported cheat mode.
* "Tech Parameter" means it's a code that does something, but really isn't a cheat, it's more or less there to help Apogee Customer Support.
* "Misc Command Line Parameter" is the same type of thing as Tech Parameter.

I've never actually seen the game crash in those situations, only remove control from you because you're perpetually being squashed. The difference is crucial: you can generally get out of those situations by typing the cheat again, and dying. Or, you can type in DNCLIP and walk away, though the warned-for crashes with DNCLIP are definitely something I've seen, so one has to exercise caution. When it doubt, DNKROZ and die.

That happens when the player character has no place to move into, use noclip to get around this.
This is the counter-part of the previous problem: if you end up within the void which results in the player having no space to "exist in", you can counter this with the godmode. I recall that there might be also different functionality between 1.3D and 1.5 with noclipping into the void.

Cant figure out how to actually enter the cheats! any help?

Just type them anytime in-game.
No console or menu, just put those cheats while standing or aiming or running or jumping...

Im guessing if I changed the key mapping it messes it up?

You mean in-game controls or DOSBox key mapping?
If it is only in-game - it does not change anything, type them same way.
If it's DOSBOx mapping - then you should press those keys, which are now associated with letters in cheat code (yeah, this one is messed).

Gotta love Ctrl+P ;)