Hey I got a few to work, the deal is that you have to type the codes in exactly right, the first time. I'm not exactly sure why but picture a console like in unread tournament or most other games with console commands for that matter. (maybe Elder scrolls oblivion is a better example)
The screen where you type the codes in can either be the screen with the "Need for Speed: Carbon" logo and the text under it, "Click to continue" or the main screen with the final bosses car (Audi TT) and all the options like career, single race, etc.
Now picture one single line on the console where the text you type goes. (but you cant see it cuz its not on the screen)
The cheat engine works by reading the exact text on that imaginary line of text. When the engine reads a pre-programed cheat in that line it activates the respective cheat and erases the text on that line so you can type in another code. One mistake and the engine doesnt read the code, doesn't activate the code, and doesn't erase the text you typed so you unable to type in a new cheat. You can not erase/delete/backspace the text so you cant fix a mistake.
Basically you have to type the codes in exactly right, no mistakes. Thats what I did and it worked for me, its hard to get em all perfect but meh. I use the trainer so i dont have to worry about it, but the times that I tried it, they went through and a screen came up with "(the respective cheat here) has been activated."