Root is the system administrators account.
If you installed in the "normal" way, you would have been asked to "supply" a root password. Though if you just kept hitting accept, you may have installed without the root password.
To see, well, on the task bar (bottom of screen) you should have an icon for a terminal/konsole/whatever you want to call it. Anyway, to access the root account, you open a terminal, and when you see the prompt of say somethin like me@mypc $ you type su (or su-) and enter, you'll then be prompted to put the password in.
If you didn't put one in during install, when it asks, just hit enter again.
the prompt changes, from me@mypc $ to me@mypc root #
that's when you can do the vi /etc/XF86config etc etc etc.
don't forget, you'll have to make sure that you know the instructions for changing stuff in VI heres a link