xdm config help

[Paul Mackinney <paul@mackinney.net>]

I'm trying to set up xdm. Whenever I boot the last message I see before
the login prompt is:

Not starting X Display Manager (xdm); it is not the default display manager.

I get the same message when I try to launch it manually as root. I'm 
been cruising the docs & all the likely files, but don't see how to set
this. I looked in /etc/alternatives, nothing named 'display manager'. I
looked at the conf files in /etc/X11, but there's nothing glaringly
wrong. There is a file named "default-display-manager' in /etc/X11, and
it's contents are '/usr/bin/xdm'.

Any tips on this issue? Last step was to apt-get update to version
4.2.1.

Thanks, PM
-- 
Paul Mackinney
paul@mackinney.net