Graphical applications use the host X server as display through a tap interface created by marionnet-daemon, so the host X server must:

1. accept tcp connections
2. allow your client to connect

Most distributions disable both for security reasons, here is how to change them:

1. change /etc/X11/xinit/xserverrc:
   from:
   

   exec /usr/bin/X11/X -nolisten tcp

   to:
   

   exec /usr/bin/X11/X #-nolisten tcp

2. $ sudo xhost +

WARNING: these changes allow  everybody to connect to your X server, so we suggest not to use them as your default!!!