You need to use Terminal and sudo nano the file. This can be done with OSX booted.
Example: Open terminal. Type:
CODE
sudo nano (the path to com.apple.boot.plist)
I don't know the path off the top of my head. Enter it in place of the stuff in parentheses.
Enter your password when asked, then use nano to edit boot.plist. It's a basic text editor that runs in Terminal.
Sudo-ing the command gives root access, which overrides the protected status of com.apple.boot.plist and allows you to save changes made.