i am not hooked up to anything like xbox live or xconnect.
I want to know if there is a way to download the extras, like missions or planes (secret weapons over normandy) to my PC, put them on a cdr/dvdr and play with them.

thanks

Nope but xbox magazine will put them on a disc usually 6 months after they are released on Live.

just make sure your regions match up- e.g. a us xb magazine's level (which would probably come out earlier at the "usual places" ;)) (ntsc) will not usually work with a pal game and vice versa