=========================================== 30/05/2013
If you have following error: Can’t connect to X11 window server using ‘:0’ as the value of the DISPLAY variable.
you must know that :

• linux user (launching process requsting Xserver) is not authorized to access Xserver
• OR DISPLAY variable is bad configured

type xhost to know:

xhost
No protocol specified
No protocol specified
xhost:  unable to open display “???”
==> means you don’t have authorization to any Xserver
==> to authorize access type: > sudo xhost +SI:localuser:youruser

xhost
xhost:  unable to open display “???”
==> means DISPLAY variable is bad configured
==> you must fix DISPLAY variable. type: > export DISPLAY=hostname:D.S

• hostname is the name of the computer where the X server runs. An omitted hostname means the localhost.
• D is a sequence number (usually 0). It can be varied if there are multiple displays connected to one computer.
• S is the screen number. A display can actually have multiple screens. Usually there’s only one screen though where 0 is the default.