Display the screen of a gui program running localy on a remote PC using DISPLAY=x:0.0

Hi,
In the case of program a running on IP-A, and displaying its screen on
a remote PC IP-B, I used to do this (before installing Trisquel I used
Xubuntu):

1) on the remote system IP_B:
'xhost +IP-A'
in /etc/X11/xinit/xservrc:
change exec /usr/bin/X -nolisten tcp "$@"
to exec /usr/bin/X -listen tcp "$@"

2) on the local system IP-A:
DISPLAY=IP-B:0:0 xterm
or
xterm -display IP-B:0.0 (or :10.0 etc)

Now I get a message that the display cannot be opened.

I need this as my system IP-B is an old PC attached to a big television
screen (used as monitor) in a meeting room.
Using this DISPLAY= option I could run my demo's on my laptop and
show the resulting screen on the big screen.
Using 'ssh -XC IP-B x2x -east -to :0.0 I could use my local mouse
and keyboard on the big television screen as well. Was very handy ...

Any help or tips are greatly appreciated !

Brgds,
Yvo.