The display manager makes your PC look like an X terminal. That is, it presents you with a login screen that runs under X.
The easiest way to automatically start the display manager on boot is to
add a line in /etc/ttys to start it on one of the unoccupied
virtual terminals:
ttyv4 "/usr/X11R6/bin/xdm -nodaemon" xterm on secure
You should also make sure that /usr/X11R6/bin/X is a symbolic
link to the Xserver that matches your video card or edit the file
Xservers in /usr/X11R6/lib/X11/xdm to specify the pathname of
the X server.
The change to /etc/ttys won't take effect until you either
reboot or ``kill -HUP 1'' to force initd to reread
/etc/ttys. You can also test the display manager manually by
loging in as root on the console and typing ``xdm -nodaemon''.