Hi, in the console, i get this error:
xhost: unable to open display ":0.0"
or
kwrite: cannot connect to X server :0.0

What the hell is this X server? I'm just trying to edit /boot/grub/menu.lst and I can't.

Any suggestion? (I tried sudo and kdesu)

As the user
Should work

The X server is the piece of software that manages your display. It acts as a "server" in the sense that all your graphical applications are "clients" which are connected to the X windowing and display system. The X server, then draws things on the screen on behalf of the client apps.

Don't immediately think of networking or internet when you hear the word "server" (although, X can run over the net in this fashion). Server/Client paradigms make sense from the high school cafeteria, all the way to the net, and even software, like X, that frequently has the client and server on the same box.

That's odd that kdesu didn't work. You might try :
xhost +
as the user- it should come back with something like
"access control disabled, clients can connect from any host"
The just su and run kwrite.

After you are finished, do:
xhost -
again, as your user (not su)
.

Thank's, it works now!