copy/paste от ето тази страница:
http://www.xfce.org/index.php?page=documentation&lang=enSetting up GDM
If you installed Xfce 4.2 system-wide and use the GNOME Display Manager (gdm) to start your X session, you will have to create a .desktop file to teach gdm about your new Xfce install. Here is a sample xfce42.desktop:
[Desktop Entry]
Encoding=UTF-8
Name=Xfce 4.2 Session
Comment=Use this session to run Xfce 4.2 as your desktop environment
Exec=/usr/local/bin/startxfce4
Icon=/usr/local/share/pixmaps/xfce4_xicon1.png
Type=Application
It is usually enough to simply copy the example file to the Sessions directory used by gdm; this directory is usually located in /etc/dm/Sessions, /etc/X11/gdm/Sessions, /usr/share/xsessions, /usr/X11R6/share/gnome/xsessions or some other location, refer to the documentation of your system for details. You need to restart gdm after you copied the file.
Lets say, your system looks for available sessions in /usr/share/xsessions, then you will need to perform the following step (as superuser) to register Xfce with gdm:
cp /path/to/your/xfce42.desktop /usr/share/xsessions/xfce42.desktop
Setting up KDM
If you installed Xfce 4.2 system-wide and use the KDE Display Manager (kdm) to start your X session, you will have to create a .desktop file to teach kdm about your new Xfce install. First, you will have to locate the directory where kdm searches for its .desktop files. The commands:
locate kde.desktop
locate mwm.desktop
may help to find this directory. Common locations are /usr/share/apps/kdm/sessions or /usr/local/share/apps/kdm/sessions. Once you found the kdm session directory, you need to create a new file xfce42.desktop in this directory, with the following content:
[Desktop Entry]
Encoding=UTF-8
Type=XSession
Exec=/usr/local/bin/startxfce4
TryExec=/usr/local/bin/startxfce4
Name=Xfce 4.2
Comment=The Xfce 4.2 Desktop Environment
If you didn't install Xfce into /usr/local, then you'll have to adjust the Exec and TryExec lines to match your installation.
Now you can login to your new Xfce desktop using kdm. If Xfce doesn't show up in kdm afterwards, check the paths you put in the xfce42.desktop file and try to restart kdm.