Accessing system after GNOME failer

I have difficulties to access GNOME in Scientific Linux 6.5

I did try to set automatic login following wiki.archlinux. In /etc/gdm/custom.conf I added:

[daemon]
AutomaticLogin=username
AutomaticLoginEnable=True

After re-boot I have main welcome screen of Scientific linux, after comes blank screen with a "working" round mouse pointer and it does not proceed to GNOME session.

I can't figure out how to access /etc/gdm/custom.conf to change added lines and comeback to a previous configuration.

Is there any way to go arround and start the GNOME, or to access terminal, or to run linux in command line mode?