I believe crtl+esc is the standard hotkey for calling up the kmenu. But you might be better off hitting crtl+alt+F1 (or F2-F6) and jump straight to one of your virtual terminals, where you can log in as root and edit the file with a cli text editor. I recommend nano for that, as it's easy to use and available by default on most systems.
The main advantage is that you can start and stop your X session without disrupting the editing of the file itself.
Use alt+f7 to get back to your X session.