On Monday 21 February 2011 21:16:27 you wrote:
> I've been doing :
> dpkg-reconfigure gdm
> and selecting kdm-trinity, and it still didn't work.

hm,
strange.

> what is the difference with xdm?

xdm is just a minimalist x-login manager (like gdm, kdm...).
normally it doesn't matter which one (from the installed ones) is configured,
as all present alternatives will be selectable.
in the end, when nothing else works than gdm, you can always 
rename /usr/bin/gdm to something else and make /usr/bin/gdm a symlink 
to /opt/trinity/bin/kdm :)
(brute force, I know).

werner