running xfce4-session-logout from script

[hernux]

Hi,

I'm trying to launch xfce4-session-logout from an acpi event script, but 
I can't find out how to do it..

this is what I'm doing now:
***
getXconsole
user=`finger| grep -m1 ":$displaynum " | awk '{print $1}'`
su $user -c "/usr/bin/xfce4-session-logout --display=$DISPLAY" 2>> 
/home/hernux/sleep.log
***
but it exits with this error:
xfce4-session-logout: No session manager running.

how can I tell xfce4-session-logout where is the session manager?? do I 
need to set up any environment var??

cheers,
hernux