In a terminal, type
Code:
sudo cp /etc/X11/default-display-manager /etc/X11/default-display-manager_backup
sudo nano /etc/X11/default-display-manager
and change it to the correct path the xdm (I don't know what that is). I know KDM is /usr/bin/kdm and GDM is /usr/sbin/gdm.
Save (Control-X), confirm (Y), and exit (Enter).