Well, the /boot/grub/grub.cfg file is a configuration file. So you can't run it as you are trying to do. You would open it with an editor, as the superuser. For example on Cinnamon you would do that with "gksudo gedit /boot/grub/grub.cfg". I don't recommend editing this file by hand, as it is generated automatically.
A better way for you would probably be to use Grub Customizer. A graphical application you can use to configure your GRUB menu. On LMDE it is a bit of work to install, but I think this post is very helpful in offering different ways to do it: viewtopic.php?f=197&t=84055